Front or rear?Totally different procedures but a Haynes manual should help.

The crankshaft seals can be replaced with the engine still in. The easier one is the front one, behind the crankshaft pulley. This entails removing the o/s wheelarch liner and unclipping coolant and power steering pipes, then the auxilliary belt(s). Undo the crank pulley, and using a seal hook, pull out the seal. Clean up the timing cover, and push in a new lubricated seal, you may have to drift it in with a large socket.

The other end is harder, because for access to it you need to remove the clutch and flywheel, after, of course, removing the transmission. You really need to get a Haynes manual to help you through.