Hi mate,
It depends which part of the aerial your trying to remove. If its the flexible aerial part, then it unscrews from the base (anti-clockwise). They sometimes get stuck, so uoi may need to use a pair of pliers or mole grips on it.
If you want to remove the base (the bit attached to the car that has the pivot on it). Then you need to do as Mk2 Dom said. Once yo removed the interior light, you will see a 10mm bolt. Remove the 10mm bolt, and then the washer which has then end of the aerial cable attached to it will slide off. Once this is done, the base simply lifts away from the top of the car.
When refiting the new base, make sure you dont get any puckers in the rubber gasket where it meets the roof. A small squirt of silicone spray on the gasket will help it to sit properly. Otherwise you can end up with water leaking in through the aerial.
If it was just the flexable aerial and it was stuck, then put a small amount of copper greese on the threads before repalcing so that it comes out easier in the future.
Hope that helps,
Alan