system
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If you are still using the Free edition, then please, you need to clarify the following:
What exactly do you mean? Maybe you allowed the AutoSandBox? If that’s the case, you need to allow the “normal” run of the program you use to edit the icons, and set Avast to accept it always (so you won’t need to allow it “each and every” time).
Anyway, I find “Avast update changing your theme/display/desktop/icons” as very unusual.
I was happier with the earlier version. Is there a way to go back to it?
Yes, use “Add/Remove programs” to uninstall Avast, use Avast Clear Uninstall Utility to “clean any remnants” and reinstall Avast Free 6.0.1203 (filehippo.com); but currently I don’t think you really need to do all that. It’s still your choice.