A simple process. As to whether you'll lose your Ranger status, I cannot say, as I've messed about with Paladins that do not have Lawful Good alignment. It didn't seem to bother anybody. Especially me.
When you open a saved game, each character is accessible. You'll find each character's alignment under the "Characteristics" tab. Change it to whatever you want. If you're worried about the moral issue of doing it, you can edit the character so that s/he is a cleric/fighter, and remove the abilities of the ranger. It isn't too difficult.