It might only be your rear pan seal; new pan gasket set will cure that.
If it is the main seal, you need to drop the pan, remove the rear main cap, push the top half of the seal (the part in the block) out with a punch - a brass punch is better than a steel one, and be real careful not to bugger the seal channel - until you can get hold of it with a pair of pliers and pull it the rest of the way out. There are several different ways to get a new upper half in place: 1)Find a "seal puller" ( I prefer this) - a braided wire sleeve, sort of like those old "chinese finger handcuff" toys;
2)Loosen all of the other main bearing caps a little bit and let the crank drop a little, and carefullypush the new seal half into the groove, then re-torque the crank. This is chancier, because of the possibility of messing up other bearings, or the timing chain.
There may be some newer techniques, but these are the only ones I have used, and this is with the "rope" style seal.