iOS: press-and-hold on a file name for a couple of seconds from the contact's Files list, and it will pop up a small menu with options. Press the Rename button and type in a different name
Mac: there are a couple of ways to rename a file:
- from a contact's Files page, right-click and select "Rename" option
- from the all Files page, select the file, and press Enter. You can also right-click, select "Show Contact Page", and rename the file from there
Note that you cannot have two files with the same name inside Contacts Journal.