how do you switch avast off
Some more details on why you feel the need to do this ?
Obvious brute force answer it to uninstall.
Other options depending why you feel the need to do this:
Right click on the Avast tray icon, select > Avast Shield Control > select the option closest to what it is you want to switch off for.
You can stop individual Shields also, go into the Avast UI > Menu > Protection > Core Shields > click the check mark below the shield you want to stop and duration.