This is the most difficult part: to open the main case you have to apply the correct pressure on the sides. I managed to open it by wriggling it a bit while pulling. Check the picture to see how the two parts are clipped, it should give you a better idea of how to manage to open it.

Gently unplug the main board from the mechanical parts, pulling it up from the bottom (it is not glued)

Remove the tape holding the cables

Unplug the water cable

You can now pull out all the components, you willl have only four cables going to the upper part (for the battery and the trigger button)

Inspect the main board

Inspect mechanical parts

The battery is glued to the upper part, you can remove it by gently pulling it from the case with a flat screwdrier