The scroll wheel is on a pivoting mechanism that allows it to perform middle-clicks.

It is best to brace the bottom of this mechanism as you're holding the frame in order to keep it steady and prevent it from hyper extending and breaking.

Insert a spudger between the left side of the scroll wheel and the housing. Then, rotate the spudger clockwise, so that it is pushing the center of the housing away from the wheel. Do this while pressing down on the wheel with your thumb.

Use firm, but gentle pressure when applying leverage to avoid breaking the scroll wheel housing.