It's been discussed here before, so information will be in Google Groups.

Basically, you do it all from inside the car. Remove the gear lever knob,
unclip and remove the gaiter from the centre console, undo the screws that
secure the centre console. (Hidden at the front under the loose covers in
the cupholders, and at the rear by a plastic plug.) Lift the console out of
the way and give to wife/girl friend/partner to clean it :-)

You can now see the mechanism and spring. Note how the spring fits. It's
quite clever how the spring does more than one job; you will see what I
mean when the new spring is fitted and you waggle the lever about.

There are four large Posidriv screws holding the change mechanism to the
car. Undo these and the mechanism  pulls apart just enough to get the
broken spring out, and fit the new one. It is not advisable, or necessary,
to pull the mechanism completely to bits.

Reassembly is the reverse etc, etc.

Spring is a fiver or less and is improved over original.

I did mine, having never seen how it fitted or read about it anywhere, in
around 15 minutes. Good luck.

Chris