I change the admin user

In 2.6.1, you can change the default admin password in the Admin Console. The URL (if you are using the default Tomcat port) would be something like:

http://localhost:8080/adminserver/

You cannot change the username for the default admin account without shutting down the server and restarting (this is in a configuration file that is only read at startup).

One last point, just in case. You can assign Presto administration privileges to any user account using the Presto_Administrator role.

For more information, see: 

http://www.jackbe.com/prestodocs/v2.6.1/prestoadmin/index.html

And search for authorization or presto built-in roles.

thanks!