This is a difficult task to do at home as you need a "special tool"
and the drive flange can be very difficult to remove.
Basically, you're going to remove the front flange of the differential, pry
out the seal, fit a new seal, and refit the flange. The flange is
sometimes stuck, stuck fast -- so you need to tap it forward with your
hammer, turning it slightly between each tap so nothing is bent or stressed.
When the flange comes off, you will want to polish the contact surface with
some fine grit paper (600 or so).

The old seal comes out with a pry bar or with a long, heavy
screwdriver. The new one is inserted and tapped into place with a hammer --
carefully!

Now comes the part that needs finesse. Reinstall the flange, fit
the nut, then begin to tighten the nut, constantly feeling the restriction
to movement of the front flange. You are searching for a pre-load, or
resistance to turn, of about one foot pound. The "feel" is a very slight
tightening. The problem here is that the flange rotates independently of
the crown wheel by as little as 1/8" on its circumference -- and you must
gauge the preload in that distance! Further, you should not use an air
impact to turn the nut as there is little control. You should use a long
1/2" breaker bar. But, you have to keep the flange from turning, so you'll
have to make up a tool that you can bolt to the front flange. I've used a
3/4" piece of rod, about two feet long, with two 5/16" holes on one end.
Run two 5/16" bolts through those holes, fix them tightly with nuts, and use
that, passed through the 5/16" holes in the diff flange to keep the flange
from turning. You could bolt this tool to the front flange, but then you
wouldn't be able to feel the preload. Tighten, feel; tighten, feel;
tighten,feel. Eventually you'll begin to draw the two taper bearings close
together and achieve this very slight resistance to turn.

I'd be pleased to review this with you on the phone. Of course, it
would make a good addition to our YouTube videos.

BTW, don't forget to refill the diff with oil!