You should be able to do it while the trans is in the car, but you need to drop the pan, and sounds like you have to remove the valve body to get at it too.
Not a trivial job, but should be hard to screw it up.
Remove the 1/2" nut on the outer TV arm and remove the arm.
Undo the 15/16" nut on the inner linkage.
Pull the manual lever out.
The inner TV rod will stay inside.
Pry the old seal out and install a new one.
Put everything back together.
There is a small "O" ring inside the manual lever to replace.
Install the valve body making sure the selector detent is in the proper position on the manual valve.
Wiggle the TV rod to make sure it's in the proper position.
Install the outer TV arm and nut. Don't overtighten the nut or you will break the inner rod. Hold the arm and tighten the nut.