My Mac has No Admin account

My Mac has just one Standard user account (no additional account else).

I've had only one account which was the administrator of this Mac.

Recently I changed the account's username from "user" to "A" as I went to Users&Groups.

However after I close the Advanced Option window, I didn't get the change.

My account name is still "user", but the account was changed from Administrator to Standard.

Now I lost all the Admin control so I can't unlock the Users&Groups and of course, I can't allow this standard account to be admin again or change the name.

I already tried to unlock with username "A" or "user" but I failed.

I guess it made some error while changing the username and took my admin control away...

How can I get the admin control back? Can anyone help this problem?