new user to avast

:-\how to switch off Avast so that I can uninstall a programme

What program are you trying to uninstall (and why, is it required) ?
There really shouldn’t be a need to do that to uninstall a program.

Right click the avast tray icon, avast! shields control, Disable fr 10 minutes, etc.
But I can’t emphasise enough that this ‘shouldn’t’ be required.