Personally, I've never run scheduled scans for many years now. So, the lack of ability to change a scheduled scan start time or days it will run in the newest version, those carried over from updating older versions of avast, is not an issue.
Aysn's advice is sound: Completely uninstall the version you are currently using, and all historic settings carried over from earlier avast installations will be lost and you will wind up with avast default settings in all areas, including any prior scheduled scans.
If you want to reinstate your schedule scans you can if you do a clean install.
Best option is to run these sort of things when you are using your computer, imo. No settings to worry about, nothing to lose or reset. Just do on-demand scans only, as needed and when needed.