Edit /usr/share/X11/xorg.conf.d/40-libinput.conf
Add Option "NaturalScrolling" "True"
For mouse:
Section "InputClass"
Identifier "libinput pointer catchall"
MatchIsPointer "on"
MatchDevicePath "/dev/input/event*"
Driver "libinput"
Option "NaturalScrolling" "True"
EndSection
For touchpad:
Section "InputClass"
Identifier "libinput touchpad catchall"
MatchIsTouchpad "on"
MatchDevicePath "/dev/input/event*"
Driver "libinput"
Option "NaturalScrolling" "True"
EndSection