Edit the following section of `/etc/X11/xorg.conf.d/10-evdev.conf`

Section "InputClass"
        Identifier "evdev pointer catchall"
        MatchIsPointer "on"
        MatchDevicePath "/dev/input/event*"
        Driver "evdev"
        Option      "EmulateWheel"      "true"
        Option      "EmulateWheelButton"    "2"
        Option      "XAxisMapping"      "6 7"
        Option      "YAxisMapping"      "4 5"
EndSection

References:

• https://bbs.archlinux.org/viewtopic.php?id=133298