Hi Thompsdw,
There are several techniques to refresh the finish of a coated garage floor. One quick and inexpensive solution is to clean very well with a green scrub pad and floor machine. Try to get it dull looking. Allow it to dry. If you use soap, make sure to rinse thoroughly! Then use a non-yellowing acrylic floor wax like ZEP that you can get at Lowes. Always read your labels. Apply 2 to 4 thin coats with a spray bottle and microfiber mop. Pre wet the mop.
That is the quick and dirty way to your goal. A more proper way is to sand thoroughly and apply a new top coat of aliphatic urethane or Polyaspartic at about 200 sf/gal. Ribbon & roll the material first and back roll 3 times. If using a Polyaspartic, thin with xylene but you'll need to contact the manufacturer to find the proper amount for their formula. I've seen ranges from 0 (not recommended) to 30%. In Utah Polyaspartic would be a fine choice but it's nice if the slab is above 50 degrees F.
Hope this helps,
Bill
P.S. At 5 years old I bet your floor is really not bad enough to hurt the sale of your house. Simply cleaning it might be enough.