There are 2 Philips screws on the back which must be unscrewed in order to remove the rear.

After unscrewing the two screws on the back, carefully lift the bottom of the rear cover and push it upwards towards the top of the device

On the left side there is a metal cover that you have to push up and then you can change the batteries

Remove the 7 Philips screws and carefully lift the plastic center piece without damaging the cables for the piezo element. Also be careful not to lose the 2 springs that are at the top right and bottom left

Now you can simply lift and remove the mainboard

The assembly takes place in reverse order.