There's usually a transparent tub with spare belts and disks marked 'belt sander' hanging around in the workshop.
0. Unplug the grinder (You'll be putting your hands on the parts that normally spin)
1. Undo the 3 slotted screws and take off the side panel
2. Undo the nut (preferably with a small wrench, but pliers will do in a pinch)
3. Loosen the Philips screw (but don't take it out completely)
4. Loosen the thumbscrew
5. Slide off the belt, if its not easy, loosen the 2 screws a bit more
6. Get a new belt, note the direction of the arrow, put it on with the arrow going in the same direction as the belt will be moving (the front part of the belt is moving down)
7. Tighten the thumbscrew
8. Tighten the Philips screw, move the belt a bit with your finger and see if it veers left or right. Adjust the Philips screw until it doesn't veer off. If it veers left, unscrew it a little bit. If it veers right, screw it in a little bit more.
9. Screw the cover back on
10. a. Make sure everything is tightened (except the belt tension locking nut)
b. Plug the grinder back in and put on PPE
c. Turn it on and again adjust the Philips screw until the belt doesn't veer off
11. Tighten the nut to lock the belt tension in place.