You change the root user account password from WHM.
Login to your WHM root account.
Change Root Passwordor type “Change Root” in the search box and then the option will appear. Click it.
Under “Change Root Password” enter your new password or use the
Generate Passwordoption. Make sure that you copy your new password in a safe place.
Click the “Change Password” button.
Your password will be changed and you can login to your root or your WHM root account with your new one.