Using a pry bar, bend the 4 metal tabs holding the scroll wheel in place away from the scroll wheel.

The scroll wheel has four plastic tabs held in place by the 4 metal tabs. After you bend all four metal tabs away from the scroll bar, lift one of the plastic tabs above the metal tab holding it in place. You may need to bend the metal tab more while you try to lift the plastic tab above the metal tab.

Repeat this for every plastic tab.