To fit or replace the belt, unplug the power cord if you have an AC model, or if you have a cordless model remove the battery.
Then take the tension off the belt by pulling back the tensioning lever and slide the old belt out.
Now, position the belt roughly in the centre of the metal backplate, and push the tensioning lever in.
At this stage the belt should fit firmly on the rollers. Once the belt’s fitted, it needs to be aligned on the rollers by adjusting the tracking so that the sanding belt doesn’t come off during use. See: How to adjust the belt tracking on a Ryobi Belt Sander