Q: How do I rename the Administrator

This went to ASC too fast, sorry.

For editing the picture:

Open Settings->Users&Groups  Password Tab:

click the lock left bottom, type your account password, then click the avatar next to the name and edit.

To edit everything:

Change the account name and home directory name on your Mac - Apple Support

Don't bother. Just create a new user that has admin capability, then delete the original while booted into the new account.

OS X isn't Windows. There isn't one magical Administrator account.*

Maybe this user has already have user data in his account.