Without loking at it, it is hard to explain.
You will need to dismantle it pretty much the same way as a Pillar Tap....
1. Isolate the water supply
2. remove the valve cover and expose the retaining screw.
3. Un do the retaining screw and remove the thermostatic cartridge.
4. Replace cartridge on a like for like basis.
5. Reassemble the valve assembly and turn water back on.
Job Done
