How can I change MYSQL username and password ( root) .NOW username is "administrator" and pass=null

Answer 1

you can, as root or with proper access, run the query
"UPDATE {dbName}.users SET password=PASSWORD({pw}) WHERE user='administrator';"

replace {dbName} with the proper db, it is usually mysql, but it may be different, and replace {pw} with the password you choose.

Answer 2

use mysql;
set password = password("idiot") where user = administrator;