You'll need to rent a floor buffer with a sanding screen, which has abrasive on both sides. Use it to take off the top surface of the finish and prepare it for the next coat of urethane. You may need to use a hand sander to smooth the areas that are already worn down. Then put down a new coat of oil-based polyurethane, probably about three coats. Apply the poly with a lamb's wool applicator and allow plenty of drying time.