Drain the oil. pry the old seal out and carefully tap in the new one. Then refill the oil. The seal has a flange on it that you can easily get a screwdriver behind to pry it out.
Put it onto a trailer and take it to the dealer for service. Just tell them " replace belly pto seal" and I am certain they will understand what you want done. Give them your name and telephone number, and turn and walk out.
if it's anything like the mid pto seal i have on my b2100 ?, i just pried the old leaking seal out, tapped new seal right back in. i did not even bothering to drain the hydro oil, i was quick !, yes..i had some hydro oil drain out ?, but it was not much, least not worth worrying about to drain all of it out. fairly easy task, and it sure beats taking it to a dealership.