Locate the 4mm screw indicated in the first picture, use a Phillips #00 Screwdriver to remove it.

Locate the two 4mm screws in the second picture, and remove those two with a Phillips #0 Screwdriver.

Now, you may remove the board that holds the scroll wheel and its components. Simply slide the wheel out and replace it.

Take note of the orientation in the final picture so that you can place the wheel back correctly into its mechanism.