If you would like to reset a password for one of your cPanel accounts through WHM, the process is very simple.
Once logged in to WHM, just select "List Accounts":
Once selected, you'll then see all of your cPanel accounts listed. To change the password, you just need to click the + button on the right hand side of the cPanel account that you need to reset the password on.
Once selected, you'll then see a box where you can enter a new password for the account. Just pop the new password in there and click "Change".
Please note that changing the password for cPanel access will also update the default mailbox password and the default FTP account password.