Take off the plastic boot trim that conceals all the rear taillight wires and what not. There is 6 or 8 (can't remember)little 10mm nuts that secure the taillights to the car. Undo all these and either have someone hold the light in place "just in case it pops out unexpectedly" or use your knee to hold in place. You will also see a male and female type plug, carefully pull these apart "remember that the plastic is most likely brittle".
Then carefully guide the unit out. Remember that the seal is most likely old and brittle and will no longer be any good to use again.
To put back in....well use your imagination!
PS. My G/F came in quite usefull when I did this, as she was the spare set of hands holding the light safely with one hand, and my beer in the other.