Clean the bearings off real well to inspect them and look at the races still in the hub, too. Since there is rust on the spindle, I would guess there might be rust or corrosion on the bearings, which may need to be replaced. Any pits at all are too many.
The seal is just pried out of the hub and the new one carefully tapped into its place. Use a metal plate if you have one, or a flat piece of wood as a buffer between the hammer and seal. Take your time. If bearings need to be replaced, use a drift long enough to pound the races out and new ones in (brass or aluminum are best, be very careful if using steel).
Sand or file off any flaky rust from the spindle, staying away from the bearing surfaces. Spray some WD-40 on to keep from rusting while you finish the rest. Clean surfaces are your friend. Wipe everything down, then clean with gas, solvent, acetone or lacquer thinner all the way up to the brake backing plate. That will keep crap from going right to your new seal.