Change cPanel Password in WHM
From Lunarpages Web Hosting Wiki
The first step in this process would be to login to your Web Host Manager (WHM) account. Occasionally, a customer will forget his password and not be able to access his cPanel control panel. You can create a new one for him in WHM.
From the first screen, click on the Account Functions. Next, go to Password Modification.
That will take you to the Change Password screen. Enter the new password into the top text box. It should be 6-8 characters and should not contain the user name. Next, select the domain for which you would like to create a new password.
The final step is to hit the "Change Password" button. Now you can advise the user of the account the cPanel and main FTP access password has changed.