You have to edit your /etc/X11/XF86Config file.
Find the place that looks like:
Code:
Section "InputDevice"
# Identifier and driver
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/usbmouse"
and add the following line to it:
Code:
Option "ZAxisMapping" "4 5"
Save it and restart the X-Server.
Hope it helps.