Unscrew the clip-on stand that connects the fan. This can be done by hand.

Turn the fan so that the back is facing up and remove the screw on the upper part of the housing.

Remove the back casing to reveal the switch's screws on the inner part of the casing. Remove those as well.

The switch's settings:

Off (center)

Speed 1 (left)

Speed 2 (right)