1. Head in to Settings -> Security -> Device Administrators.
  2. Un-tick the box next to ScreenLimit and say OK to removing as a device administrator.
  3. Go back to the ScreenLimit app icon and long-press it so you can Uninstall the app.
  4. To completely remove the settings for the device, also remove it from the Devices list of your ScreenLimit Dashboard (using the cog beside the device name).