How can I change my administrator log in user name

Changing the name of any user login account is risky. It can be done safely IF you read and understand the guidance that Apple provides here: Change the name of your macOS user account and home folder - Apple Support

Read it completely three times before you attempt it.

HOWEVER, before you attempt this, make sure you have backed up your data via Time Machine or other means.

ALSO, it would be wise to create an additional user account that has administrator privileges on that Mac. If something goes wrong in your process it can be used to help you get out of trouble.

Add a user or group on Mac - Apple Support

If this is a new-to-you Mac, you don’t really want to follow the login name rename path.

You want (need) the owner to relinquish ownership.

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Otherwise, ~everything will be owned by the original Apple Account, and not yours.