To rename a file using OS X (Sierra) begin by selecting the file you want to rename; Either click on its file icon or file name. Nothing will happen at this point as the click only selects the file (highlights it in blue and makes it the focused/active object). This is quite normal.
Fig 1.0 Click on a file in order to select it (highlight it in blue and make it the focused/active object)
Now click on the name of the file (its actual file name). Doing so will place that file name inside an edit box, ready for editing/modifying - Renaming in this case. At this point you could press any key on the keyboard to erase/delete the whole of the existing file name, apart from its extension (i.e. .docx). So if you press the DELETE keyboard key you will be left with a blank file name (i.e. just .docx - Fig 1.2) inside the edit box whereas if you press the letter H you will be left with the H inside the edit box (i.e. H.docx) which is good if your new file name begins with H; Press the first letter of your new file name in other words instead of the DELETE keyboard key.
Fig 1.1 Now click on the name of the file (its actual file name) in order to edit/modify it (Rename it)
Fig 1.2 Press the DELETE keyboard key to delete the whole of the file name, except its extension (i.e. .docx).....
Fig 1.3 .....or press the first letter of your new file name (i.e. H) to delete all of the file name, leaving you H.docx
When you click anywhere inside the edit box for the first time the text becomes unhighlighted whereby the background of the edit box turns white and a flashing text cursor appears where you clicked; where you positioned the mouse pointer. So with the original file called Wedding_Plans (Fig 1.0 above); If I click to the left-side of the letter P, or to the right-side of the Underscore depending on your point of view, pressing the DELETE keyboard key will delete the letter P whereas pressing the BACKSPACE keyboard key will delete the Underscore character.
In the example below I have clicked in between the Underscore and the P, which is effectively to the left of the letter P and to the right of the Underscore character, and will now delete the word Plans and substitute it with the word Vows so that the existing file name of Wedding_Plans (Wedding_Plans.docx) is then renamed to the new file name of Wedding_Vows (Wedding_Vows.docx).
Fig 1.4 Click in-between _ and P
Fig 1.5 Delete word: Plans
Fig 1.6 Type word: Vows
Fig 1.7 Press ENTER
When you rename a file you usually press the ENTER keyboard key to set the file's new name. However, this also reselects the file and its new name; which means accidents can happen. For example: If you press the DELETE (CMD + BACKSPACE) keyboard key without thinking or realising it, while the file is selected, the file will be deleted straight away (moved into the Trashcan). While there is no real danger to this on your computer, because you can simply undelete it by using the PUT BACK menu-item on the Trashcan's context menu (right click pop-up menu), deleting an e-mail file or network file by mistake may mean it cannot be undeleted/recovered. Now imagine that file has valuable wedding guest contact details and/or wedding itinerary notes inside it.
OS X (Sierra) does not normally display a message requester like Windows 10 asking you "Are you sure you want to delete this file?". This is in part because OS X (Sierra) knows it would virtually be impossible for you to accidentally delete a file when you have to press down two keyboard keys in the first place to do so - CMD and BACKSPACE. Regardless of this though. To avoid this scenario altogether and to set (apply) the new file name just double click anywhere away from the newly renamed file (i.e. click inside its folder's display area / window). The first click will set the new name but leave the file selected, hence why you need the second click to deselect the file. This is assuming the file is inside a folder and not on the desktop for example.
Fig 1.8 Double Click anywhere inside the white display area of the DOCUMENTS folder to set the new file name
Another thing to be aware of when you are inside the edit box, renaming a file, is that you can also use the LEFT and RIGHT Arrow keyboard keys to move the flashing text cursor left or right. This is good if you want/need to reposition the flashing text cursor in order to rename a different part of the file name. So if you are renaming Wedding_Plans to Wedding_Guests and rename it to Weding_Guests by mistake, there is no need to delete or backspace out ing_Guests just to insert the missing d. You could just move the flashing text cursor to the left-side of the letter i (to the left-side of ing_Guests), using the LEFT or RIGHT Arrow keyboard key(s), and then type in an additional letter d.
File Name Extensions (i.e. .docs, .rtf, .mp3, .txt. jpg, etc) are normally switched off (hidden) by default. This is because deleting them by mistake can have side effects for those type of file. Normally it will not be or cannot be opened by its default application simply because the default application will wander what data is inside a file called Photo for example when it has no .jpg or .png file extension to give it a clue. This is because many applications insist on only opening a file that has a known file extension attached (suffixed) to its file name. So if you rename Wedding_Plans.docx to Wedding_Vows for example normally OS X (Sierra) will complain with a warning that states the file might not open with its default (standard/natural) application if you remove the file's extension.
Fig 1.9 Do you really want to remove the .docx file name extension from this file?
Years ago a file was identified by its file name extension only. So if a file did not have an extension the application would play safe and say "I'm not opening that file. It looks alien". These days applications are more intelligent and read a file's data, as opposed to its file name extension, to assess what type of file it is. A Web Page file is an example of this:
Years ago a web browser would insist on a HTML file (web page file) having a file extension of .html before it could be opened whereas these days the web browser doesn't really care if it opens a file called Page.htm, Page.html or even Page because it would know the file's data consists of HTML code (text data) as opposed to Music or Photo data. The same applies to a photo package. Regardless if a photo file is called John.jpg or John it makes sure the file contains photo data before opening it.
Saying the above: If you want to play safe and avoid accidentally deleting a file's extension (even though the above warning would help you prevent that from happening), you should untick the SHOW ALL FILENAME EXTENSIONS setting in the FINDER Preferences window (ADVANCED Tab).
Fig 1.10 Click on the FINDER menu and select its PREFERENCES menu-item to continue
Fig 1.11 Untick the SHOW ALL FILENAME EXTENSIONS setting to disable the showing of file name extensions
Disabling (switching off) the display of file name extensions is not always a good idea because as you work more with folder and files you will begin to see the advantages of being able to identify that a file is indeed a .jpg file and not .png file for example. Likewise; that a file is a .docx file and not a .doc file. Many files can be determined and therefore identified by their unique icon of course, but when icons and file names look similar within a folder full of files you will know whether or not enabling the SHOW ALL FILENAME EXTENSIONS setting is right for you.
Personally, I have this setting switched on (enabled) simply because of the just mentioned reason. I like to see/know the file type of each file listed, especially with MS Office files (.doc vs docx) and photo files (.jpg vs .png).