Once the clocks are out they should look like this:

The black plastic cover just unclips to reveal:

Note the 2 blue twist fit leds in the middle of the clocks, these illuminate the lcd screens, also there are 2 more leds at the top of the unit, these shine down and illuminate the dials themselves.
Simply remove the old led's and replace, you can see in the pic below the red ones that I have replaced them with. If they don't work, then don't worry, remove them and replace them the other way around.

Put them all back together and you are left with:


I've also got a few pics of another style of clocks which i'll post up later!