To change your wheel you need to remove the guards from both wheels, when doing this make sure you use a screwdriver that is a good fit as the cheaper machines are using very soft metal for these screws. Then you need to put a socket or spanner on each nut. To free the nut it is best to tap the spanner in the direction of rotation of the stone, which is usually towards the rest, there should be an arrow on the machine to give rotation direction. When fixing the stone don't over tighten, firm but it's not a test of strength. The machines from new tend to come with sized holes whereas replacement wheels have a 31.75 hole and reducing bushes. The sizes are generally imperial but are often listed as metric, measure the hole, the most common sizes are 12mm 1/2" (actually 12.7mm) 16mm 5/8" (actually 15.88mm) 19mm 3/4" (19.05mm) 22mm 7/8" (actually 22.225mm) and 25mm 1" ( actually 25.4mm) there's also 20mm on occasion.