Where to start?

How to remove the case? There are so many different small screws, I do not understand what you need to unscrew.

You need a screwdriver with a long thin blade to disassemble the device. Generaly all case screws marked in the documentation, so you can find them easily.

Should I switch off the device before disassembling?

Switching of is necessary. The device may contain parts of circuit with voltage more than 15 kV.