If you have already created a Mysql user under cPanel -> Mysql Databases section you may want to update its password.

You can complete the task through:

cPanel -> MySQL Databases

Find the MySQL Current Users list and click on the respective user.

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.

If you do not remember the old password you should delete the user and create 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.