how to replace rear seal in motor

There are a few possibilities. Rear main seal, oil pan gasket or it could be from a leak up higher on the engine (valve cover gasket, distributor o-ring). If it's automatic transmission fluid it could be the pump seal on the transmission. If it's engine oil and it leaks a lot while the engine is running, then this would be the rear main seal. If it's an occasional drip, then it could be any of the seals. To inspect the rear main you have to remove the metal bracket under the back of the engine, then you can see the rear main seal to see if that's the leak. Make sure the PCV system is not plugged, this will cause all the seals to leak.