I can’t uninstall or modify, or close Avast Premium Security after the recent update. I can’t use the task manager to end Avast either.
In normal mode, the self-defence module would prevent you disabling/closing an avast process.
Download the avast! Uninstall Utility, avastclear.exe https://www.avast.com/uninstall-utility and follow the instructions given on that link.