When you say avast can’t update, we need to know why, e.g. what errors were displayed, etc. That gives us a clue as to what to suggest.

The fact that avast can’t update doesn’t mean you have no antivirus.
Is the avast tray icon present and occasionally do you see it rotate (indicates it is scanning), are there any other signs over the icon like an ! or an X ?

Check the task manager are the avastSvc.exe and avastui.exe shown ?