It's not hard-- you remove the door trim, peel up the water shield, undo the three bolts that hold the motor on the regulator, disconnect the motor wires, install the new motor and reassemble the door. A Haynes manual (from autozone, etc) will give you pictures and instructions. For the rear doors you may have to drill a hole in the inside sheet-metal to access the third bolt for the motor-- there's a dimple in the door that marks where you need to drill.
Before you buy parts and do all that work though, make sure the problem is not electrical. The window switches are a weak point and worth testing. You can test the electrical by seeing if you get 12v reversed polarity for up and down at the motor connector's on each of the doors when you operate the switches.