There are a few how to's around the net for doing it, not real hard.
Basically you need a drain pan for the old fluid, w 3/8 breaker bar (to remove the fill plug, it is in there tight), I think a 12 mm socket for the bolts on the diff, gear fluid (and Friction modifier if you have a limited slip) and some gasket maker (I used Permatex Ultra Black, but there are other options).
Some drain it warm, others cold. I did mine cold.
Put the pan under the rear axle, loosen all the bolts on the diff cover 1/4 a turn. once all loose, start removing them (bottom to top) leaving the top 3-4 in. If it has not started draining, take a small screwdriver and pry it apart and the fluid will spill out.
one note, gear oil stinks horrible, so wear nitril gloves if you have them.
Once it is all drained, take the rest of the bolts off and remove the cover. clean off the inside of the cover (I used brake clean to get the oil residue off and a plastic scraper for the gasket maker stuff). get all the sealer off the diff housing itself also. Now get a few rags (or a gloved hand) and "scoop" out the oil in the bottom of the diff, there is about a cup of oil down there that you want to get out.
Now clean the diff sealing area again and wipe it off clean. Apply the gasket maker (several options out there, just follow the directions on it).
With the ultra black, I applied a 1/8" bead all the way around the diff cover, with a little bead around each bolt hole also. I let it skin over (10 minutes or so) then installed the cover finger tight. Let it sit for about 20 minutes then tightened all the bolts in a criss cross pattern (did not use a torque wrench, just did it GnT (Good and Tight)).
Removed bolt from fill hole using 3/8 breaker bar and then fill the diff with gear oil (2.75 quarts I believe). Basically fill it till it runs out the hole. If you have a limited slip, add the friction modifier to the 1st bottle after it is 1/2 in, much easier to do that way (and the FM sticks much, much, much worse than gear oil). once full, put the plug back in, clean up and your done. Take it for a test drive and look for any leaks.
I may have missed something but this is the basics of it.
: Here is a how to with pics
, say basically what I just typed out with a few small differences, but nothing major.