You can change the root user account password from WHM.
Login to your WHM Root Account.
. Go to Server Configuration
>> Change Root Password
or type Change Root
in the search box, and the option will appear. Click on it.
Under Change Root Password
, enter your new password or use the Generate Password
option. Make sure that you copy your new password in a safe place.
Click the Change Password
Your password will be changed, and you can log in to your root or your WHM Root account with your new one.