Unplug your dryer and move it away from the wall.
Insert your putty knife under the top panel of the dryer at the corners. Pry up on the knife to pop the panel up. Lift the panel so it's standing straight up.
Unplug the wiring behind the front panel of the dryer.
Remove the mounting screws on the rear corners of the dryer's front panel. Remove the front panel. You now have access to the belt, which is wrapped around an idler pulley.
Check the belt for signs of damage. If it's twisted, simply untwist it so the grooves on the belt are facing the drum. If the belt has come off due to damage, you'll need to replace it.
Remove the belt from the pulley and fit a new one in its place. Make sure it isn't twisted over the pulley.
Replace the panels.