I know you probably didn't mean it the way said it, but the seal doesn't actually fit to the drum, it fits to the static part of the washer just in front of the rotating drum.
First you need to remove the top of your washer and carefully remove what you can from the top of the inside such as the water feed to the deturgent tray, cross strangtheners etc. I found it best to take a few photos with your phone to remind yourself where it all goes back. You should be able to remove the control panel from the top of the front panel, there will probably be a couple of wires still attached but they are long enough to get it out of the way. With the door open, you should be able to remove the front seal retainer and then the seal from the front, once removed you can access the rear of the seal and the retaining spring (device). Clean everything up with some soapy water and fit the new seal, simples!
You can move or hold the drum back slighly (3 to 4 inces) to give you a better view, you may need another pair of hands, but you have to work from the top of the inside of the washer, it's tight! Just take your time and double check the positioning of the seal (drain holes at six O'Clock) before closing up and reffiting everything you removed. Good luck - it's not that bad a job really and if you get really stuck, you can always call out an engineer, at least you'll have a new seal!