Hi Mary,
You need to remove the plastic covers at the back of the machine.
You will see the timing belt; the ends are clipped together. Undo the clip, it may be easier if you move the belt so that the clip is at the front and carefully remove the timing belt. Be careful you don't cut yourself. Thread the new one into position and join the ends. You can slide the belt round so that the join is at the back.
Hope this helps, I once replaced one on a KH890.
Sue
February 2011