For shields with the Action option:

Open Avast! > SECURITY > ANTIVIRUS > File System Shield > Settings > Actions See image

Be aware that Web shield default and recommended action is to Abort Connection.

For Scan computer:

Open Avast! > SECURITY > ANTIVIRUS > Scan Now > choose your scan of choice > More details > Settings > Actions.

Click " Automatically apply actions during scan " choose " No Action " and then unclick " Automatically apply actions during scan ". So next time Avast! finds a malicious file, it will open a window with choices of what to do with it. see other images