Gas lawn edgers use a drive belt that runs from the engine flywheel pulley. The belts wear due to heat stresses and use. It's a good idea to replace the belt if you see signs of rotting, cracking or shredding. The belt must be replaced if it's broken completely, and the edger won't work if that's the case. Replacing a belt involves noting the correct orientation, adjusting a fastener and adding a new belt.
Things You'll Need
Cellphone with camera (optional)
Stand in front of the edger and take a look at the existing orientation of the drive belt if it's still on the lawn edger, and take a photograph of the routing with a camera. Cellphone cameras are good for this. This will help you put the new drive belt on correctly. You can also draw a sketch by hand.
Loosen the flange nut on top of the frame, letting the idler pivot assembly pivot a little. Reach into the back of the edger and pull off the existing belt from around the flywheel pulley. Then remove the spindle belt guard by removing any screws you see there. Remove the belt off the further pulleys and throw the belt away.
Place the new belt onto the spindle sheaves and route it back through the pulleys using your photograph as a guide. In all cases, the belt has to be on the engine flywheel pulley and the idler pulleys. The "Y" on the belt seats into the top pulley and the flat part goes into the bottom. Re-install the spindle guard and re-tighten the nut on the frame that you loosened.