You can use server’s root password to login to any cPanel account. In order to reset cPanel password for a website, you should know present password for it. But, of course you can change the password from WHM control panel. Here are the steps to do it.
1. Login to WHM. http://<server ip address>:2086/
password: <root password of the server>
2. Home –> Account Information –> List Accounts
3. Click on “+” sign near the domain for which you want to change the cPanel password. You can see an option there to change the password.