As far as I know, the update function isn't yet available.
Thanks bob3160 You are correct
I downloaded version 65.2.491.181. and installed over the top.
Would be nice if avast opened a thread when Avast Secure Browser is updated.
There used to be a topic where avast posted updates and changes.
At the moment I do not have a clue if avast has updated their browser or not. So hope they hurry up and get the built in updater to work.