How can I change the privileges of phpmyadmin users without knowing the root password?

Question

PHP

Good afternoon, there is a server on nginx, the phpmyadmin DBMS is installed there.

Created the testuser user there and limited privileges.
root is not.
The question is, how can I set ALL PRIVELEGES without having a root user from phpmyadmin ?
There is only access to the server on which it's all installed

Please describe all the options that you know.

Thanks