It's often a fiddly job rather than a tricky one.
The exact method of doing it varies from machine to machine type, but the basics are the same.
there will be a circular clip on the outside behind the door that has to be removed first and then a large clip right the way round the inside lip where it attaches to the outer drum to be removed, then remove the rubber.
when putting the new one on make sure it is fitted properly over the lip on the inner drum, refit the clip then do the same for the outide on the front of the machine.
with hotpoints it's often easier to remove the entire front panel of the machine to do the job.
Be careful following gulliver1's advice of turning it round as some door rubbers have a vent hole in them at the top and if you turn it round then it will leak through the vent hole, though if it only has a small leak and no vent hole then turning it round 180degrees is an option.