RE: Modify Label in Dynamics 365 with x++.
If different users (or users in different companies) should be able to set different values, changing the label wouldn't work anyway, because it would change for everyone.
If you manage to change the resource file (which you can't), you would achieve very little, because the change would be lost at the next deployment of the package.
I wonder what's the actual business requirement that you decided to solve by this technical solution. Maybe they should simply request a minor change and get it delivered in the usual way, assuming that you own the label file and can make changes. If not, you may also need to update properties to use a new label ID from your own package. Or maybe you need something more complicated, e.g. if you want different users to set different values.