Do I need to remove the drum to replace the belt? If so, how do I do that? Do I go through the top?
No you should not need to remove the drum.
You will need to remove the front of the dryer and the back access panel.
You can slip the belt over the drum, and through the back access panel you can place the belt over the tensioner and then over the motor pulley.
Good luck, let us know if you need any more help.