I would be grateful if anyone could point me to the right
way to get it off.

I had exactly that model. I seem to recall some difficulties working out the
securing mechanism, so I removed it with a lump hammer. Unfortunately, I
can't remember the "correct" way to remove them, although it was obvious at
the time from the remaining pieces.

However, you should note that it isn't a thermostat, but a central heating
programmer. You might also want to do some sort of fault finding
investigation, rather than just replacing components randomly.

Christian.