Very important ! Just stick to the instructions in the tutorials.
How to change Email password in cPanel RoundCube Email UI
Once logged-in, click here
Now click here
Now fill in new password and click 'SAVE' and password will be changed.
Note:- Choose a strong password and Follow the password rules while changing the old password.
Once password changed, click here to go back.
Now click here and you will be able to go back to your main mailing screen.