There's a page in WHM that allows you to manage all the MySQL passwords on your system. This is useful because you don't need to know an existing password to change it to a new one.
This tool will allow you to change the passwords used by any system programs that need a database, such as roundcube. You will probably never need to know, use, or change these passwords, but it's good to know how, just in case.
1) Go to the SQL Services menu.
2) Click Change a User or Database Password.
3) Select the MySQL user from the list. For system services, the username should reflect the application that uses it.
4) Enter and confirm a new password, or use the Password Generator.
5) Click Use Password.
6) Make note of the password, if you'll need it.
7) Then, click Close.
8) When finished, click Change Password.
That's it! The user's password has been changed.
This completes the demonstration. You should now know how to manage MySQL user passwords in WHM.