If there's only one account, it is automatically an administrative user, and you cannot change this unless there are multiple accounts. There has to be at least ONE administrative account.
So why don't you tell us a bit more about just how you went about changing the name on the account?
"I have macbook pro, yeaterday i change the user name and set the password, after i restart it is not allowing me to open anything, i think i dont have admin access, and in log in screen i have only 1 account, how can i go to admin user"

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