Remove the old belt from between the white wheel on the left and the small black motor spindle wheel towards the middle.

Clean the grooves in both wheels using some IPA or other cleaner. Make sure to remove any residue from the old belt. For the white wheel, a good way to do this is to insert a cotton bud or into the groove and slide the tray in and out gently, as this will spin the wheel quickly.

Leave the wheels to dry, using a cloth if necessary.