I don't know this machine personally, but usually the wheels are held on by a circlip which you remove, and the wheel just pulls from the axle. However they frequently rust on, so apply a good releasing spray such as WD40, let it soak in, and try to work the wheel free. You'll see a square woodruff key in a slot which locates the wheel onto the axle when it does come free. Don't lose this key!
John
May 2009