Grab the mouse on both sides and push with your fingers so that the case reveals a small gap

Insert the metallic spudger at the back (see pic 2) and slide it along the edge of the case.

Don’t insert it too much (see pic 3) there are components on the way

You also need to insert the spudger at the front. As pic 3 shows, there are two plastic pins holding the case in place at the front. (See the markers)

that should open the mouse.

Be careful with the cable connecting the click buttons to the motherboard! (See pic 3)