Paxe, a word of warning before you do this the way P6MAD has suggested. If you support the car on the crossmember the suspension will hang down. Then when you remove the top damper mounting the force of the roadspring will force the lower link downwards. If you have an assistant down there with his head next to the lower link,now's the time to phone him an ambulance!
Far better to run the rear up on to ramps, then just drop the damper out.
If you want to do it by jacking, remove the top damper mount BEFORE you jack up the car. You will then need to support the car on stands so you can jack up the bottom link to fit the new damper. The damper will not be long enough with the suspension all the way down.