Step 1: Here we have a couple of still-together mice
One of these says "version 3" on the bottom sticker, and the other doesn't. They're USB mice, although apparently they'll talk PS2 as well, using a merely physical adapter (these adapters don't actually translate between PS2 and USB protocols, they just connect the wires differently. The processor inside the mouse detects which kind of connection is in use, and reconfigures its pins as appropriate.)
Step 2: Find and Remove the Screws.
Step 3: Remove Mouse Guts
The two mice, despite being outwardly "identical", had significantly different internals. The one in the photos here had three circuit boards, and some additional screws to remove. The other (older?) mouse only had two boards, and was had its internals held together entirely by plastic tabs and slots.