you may want to consider why the tube went flat as well. 20 year old tires are not exactly reliable or safe.
you can get new tires from most any motorcycle dealer (through tucker rocky catalog) for about 12 bucks each.
as to how you do it, search the web on how to change a bicycle tire. to get the wheel itself off, take the nut off the wheel, pull the fender, and slack the brake cable it should come right off. if you dont feel comfortable doing it yourself it probably wont cost much to have a dealer do it.
randy