Changing a macOS username isn't as straightforward as just changing the user's name--care must be taken to ensure permissions and file access rights aren't broken in the process. Follow these steps.
Many macOS users learn from experience that it's a bad idea to just rename a Mac user account. Sometimes users seek to use a nickname, a different user begins using the computer, or maybe a user decides he or she would like to use a different name for his or her Home directory. Due to the way permissions, file access rights, and other operations run in the background, operations can go irrecoverably wrong if an error is made. Here's the process Apple recommends following when renaming a user and Home directory within macOS.
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Begin by making a backup. It's always a good idea to ensure you possess a complete, verified Time Machine backup before making any significant system changes. Changing the primary user name, or renaming the primary Home directory, are two good examples. Make that backup, check it, confirm it's good, and then proceed.
Log in using the administrator account, which must be a different account than the account you wish to rename. If you're changing the name of the administrator account, if there's no other admin account on the Mac, you must create a new admin account because you can't rename an administrator account while you're using it.
Navigate to the Users folder on the startup drive. Within the Users folder you'll find the Home folder for the user account you seek to rename--don't use any spaces when you rename that account's Home folder. I recommend writing down the old and new names, just to be safe.
macOS will prompt you to enter the administrator username and password. Do so, and then proceed.
Next, click the Apple icon on the menu bar, select System Preferences, and select Users & Groups.
Click the padlock and enter an administrator username and password to enable making changes.
Review the list of users, and then control-click or right-click the user account you wish to rename. Choose Advanced Options from the pop-up menu that appears.
Edit the Account Name (also known as the short name) field (Figure A) to match the new name you gave the Home folder in the Users folder when you renamed it. Check your notes to confirm you have it correct; again, avoid using any spaces when spelling the name.
Edit the Home Directory field to match the new name you provided the Home folder within the Users folder.
Click OK and close System Preferences. Restart the machine, log in using the renamed account, and verify the old Home Directory contents are present.
These same steps also work for OS X El Capitan, Yosemite, Mavericks, Mountain Lion, and Lion releases, incidentally.
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If you wish to change just a user's full name when using macOS (or OS X El Capitan or Yosemite releases), click the Apple menu. Then select System Preferences and click Users & Groups.
Click the padlock icon found in the lower left corner of the window. Supply the administrator username and password to continue.
Review the list of users. Control-click or right-click the user account you wish to rename.
From the pop-up menu that appears, click Advanced Options.
Change the name that appears within the Full Name field. Review the name carefully for accuracy, and to avoid confusion, ensure the name doesn't match or too closely resemble a pre-existing account already present on the Mac.
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