If you have already created a MySQL user through the cPanel -> Mysql Databases tool you may want to update its password.
You can complete the task through the same tool -> Current Users section. Click on the respective user and a new MySQL Account Maintenance page will be opened.
There you can change the password by entering the old password in the first field and the new password into the next two fields. You can also use the built-in Password Generator.
In case you do not remember the old password you can delete the user and recreate it again. Remember to add the user to its respective database once it is recreated. Detailed instructions on how to complete this task can be found in the following knowledge base article: