MX500

/etc/X11/xorg.conf

Section "InputDevice"
      Identifier  "Mouse0"
      Driver      "mouse"
      Option      "Protocol" "auto"
      Option      "Device" "/dev/input/mice"
      Option      "Emulate3Buttons" "false"
      Option      "ZAxisMapping" "6 7"
      Option      "Buttons" "7"
      Option      "Resolution" "800"
      Option      "MaxHWRepInterval" "100"
EndSection

~/.xprofile
(check where the 'xmodmap' is, it may be /usr/X11R6/bin/xmodmap for some cases.)

/usr/bin/xmodmap -e "pointer = 1 2 3 6 7 4 5"