You can edit the file /etc/X11/xorg.conf (sometimes called XF86Config).
Find 'Section "InputDevice"'
I use this protocol
Option "Protocol" "IMPS/2"
and most important change to
Option "ZAxisMapping" "4 5"
You might also be able top use "6 7" if you have more buttons. Try what works for you.
Backup the file before you save your changes!
Then you need to restart X. Logout and press Ctr-Alt-Backspace.
Good luck!
E