WARNING: Be careful in this section. There are two almost microscopic springs that are part of the mouse wheel. Do NOT turn the mouse up side down after removing the locking pin or you will LOSE them.

Rotate the free spin activation arm up (as can be seen in the first picture).

Make a note of the orientation of the locking pin, curved section facing down and slide it out of the socket (as can be seen in second picture).

Once the locking pin has been taken out, the entire mouse wheel should lift out of it's resting cradle. Make sure you note the springs (can be seen in the third picture).

Do not lose these springs as they serve to give your middle mouse click that "bounce". They are almost impossible to find if dropped. (Yes I did drop mine and I found them again through sheer dumb luck)

Store locking pin and springs in a safe location.