I would advise that before you do anything, you get all that area cleaned up first so you can see where the oil is leaking from. I say this, because I noticed from your photo that you have oil above the diff also; so you could have a leak from somewhere else. Check your power steering box for leaks. Check your oil level in the diff straight away just in case, or you will be looking at more than a seal.
To replace the diff seal undo the propshaft bolts at the diff end,there are special spanners for the tight position of the bolts, but a good fitting double ended spanner will do, and swing the propshaft to one side. Drain any oil from the diff.
You will then see a large nut with a split pin through the thread, remove the pin and get the correct socket for the nut. This nut is very tight so you will need a breaker bar or use your jack to release the nut. Undo the nut and pull off the spider, a soft mallet helps here.
Get a hook tool and pull out the old seal. Tap in the new seal just slightly below the edge of the diff make sure the flat part of the seal faces outward. Bolt up in reverse order. Fill with new oil.
You can do this very easily even without jacking up the car very high. You need the weight on the wheels and the hand brake on, so jack the chassis.