Remove the kick starter from the shaft. Pick the seal out, press the new one in. Re-install the starter onto spline.
Worst case scenario you can't get the old seal out you might need to drop the oil and the coolant, remove the rhs engine cover. Seal is part of this cover. Then press the old one out, and press the new one in. Done right you'll likely be able to re-use the rhs gasket too, although I'd have a spare on hand and determine how the old one looks at the time.