Starting point:
$> grep -A5 TouchEdges ~/.config/kwinrc
[TouchEdges]
Bottom=ShowDesktop
Left=None
Right=None
Top=None

Systemsettings -> Desktop Behaviour -> Touch Screen
set "Top" to "Application Launcher"
hit "Apply"

$> grep Top ~/.config/kwinrc
Top=None

go back with - All Settings -> Desktop Behaviour -> Touch Screen
set "Top" to "Lock Screen"
hit "Apply"

$> grep Top ~/.config/kwinrc
Top=ApplicationLauncher

go back to with "All Settings"

$> grep Top ~/.config/kwinrc
Top=LockScreen

Also the settings are only propagated to kwin if the dialog is left via "All Settings" -> quit, not when quitting the systemsettings directly using the "(x)" button in the decoration.