Well here is how you change permissions using security setting.
Click on Permissions to open the "Permission for Appx" window. Click on Advanced to open a window like this:
Currently, the owner is TrustedInstaller. Click on Change. In the Select user or group window, click on Advanced..., the click Find now. You will get the list of all possible user/groups on your computer.
Select Administrators (the one with 's'), hit OK, hit OK once more. Make sure you select "Replace owner on subcontainers and objects", then hit OK once more. You should be back at the "Permission for Appx" window now. In this window, select Administrators and check "Full control" box under Allow. Hit OK the last time.
Now you have full control of the registry Appx folder and will be able to change every key inside this folder as you wish.