Easy seal to change:
1 - Drain a couple gallons from the transmission to lower the oil level. Or drive the front end onto some blocks so the front's 18" or so higher than the rear.
2 - Remove (unscrew) the drive coupler. Stick the transmission in low, or engage the PTO then hold the PTO shaft from turning with a pipe wrench. Now grab the coupler with a big pipe wrench and give it a heave (it's normal right hand threads). Once started the coupler will spin off easily.
3 - Remove the capscrews holding the seal retainer then pry it off.
4 - It's now a simple matter to remove the old seal and press in a new one.
Note the new seal will be about 1/2 the thickness of the original you take out. This is NOT a problem. Simply press it in flush or a bit further and it's good to go.
Reverse the process to put it back together. No need to wrench tighten the coupler - spin it on finger tight and it will tighten itself the first time the pump is loaded.
Tip: Consider replacing the upper seal while you are in there! Can almost guarantee if the bottom seal is worn out then so is the top one.