Removing the rear wheel shouldn't affect the derailleur. Follow Rider77's instructions and it should drop right out. Depending on tire width, you may also need to release the brake tension to clear the tire. It can be a little fiddly to re-install, but it's pretty hard to really mess up anything. If you don't hit the correct cog with the chain just spin the crank with no load and it will alighn itself. Don't be afraid of it. Practice a few times and you will see how easy it is to R&R the wheel.