When you open the Avast GUI > Settings > Updates > Programs > is it set to Automatic or Ask or Manual?
If it is set to Automatic, it will update automatically.
If it is set to Ask, you will get a red indicator in the main page of the Avast GUI that there is an update available, but you have to click on the "update" button to update it.
If it is set to Manual, you will have to find out when there is an update, and do it manually yourself...not suggested.