Not hard but it is annoying... Get a friend around so they can raise and lower the aerial while your holding all the bits
I can't be too specific though as I haven't done it for years.
Turn you wheels to the left to give you access to the black plastic guard liner inside the left wheel arch OR take the wheel off.
Undo heaps of the screws holding the liner in, some will be metal some will be plastic clips.
Pull, yank, fight the liner out towards the middle of the car and get your arm in there.
Feel around for the motor and find the very top.
You MIGHT be able to squeeze the tab holding the black surround for the aerial hole and pull it out and off the aerial. If not go to '***' below.
IF you can, then use a pair of long nose pliers and undo the bezel on the top of the aerial through the hole where you pulled out the black trim bit. It will be a pain and you'll probably have to push the motor bit up as high as you can to get the pliers on the bezel.
If you can't get pliers in there then there should be a rubber mount or strap that you can undo which will allow the motor to come up and out of the hole then use a spanner to undo the bezel. WATCH YOUR PAINTWORK...
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If that doesn't work then undo the strap/mount and wiggle the motor downwards and around so you can get it out from behind the guard and into the wheel well.
Undo the bezel with a spanner or big pliers.
Raise the aerial and pull the bugger all the way out, mast and rope/plastic bit. It will sound nasty but just do it.
Put your new aerial rope bit in with the serrated bit facing toward the motor and put the aerial down. It will be hanging out too far but you'll fix that next-
Screw the bezel back on and raise the aerial again. It will clank and clunk a bit but that's ok. Lower the aerial again and it should be set right.
Poke it back into the guard and up into the plastic surround thingy and do up the mount/strap. Test it before you put the rest of the stuff back.
Fingers crossed, it's all good.
If anyone else thinks of something to do with this please add!