You can change the root user account password from WHM.
1. Login to your WHM Root Account.
2. Go to Server Configuration >> Change Root Password or type Change Root in search box and then the option will appear. Click on it.
3. 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.
4. 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.