EDIT: 10/7/20 -This article is SUPER old and things have changed since then.
This is the official and most recent method:
Double tap on where the autofocus should be calculated. Pick an area of high contrast.
You can two finger tap on the camera preview to force it to do a focus update. This is done at the point directly over the takeoff location while the aircraft is not moving.
There is currently a bug that makes it so it isn't updating the focus after the first flight in a multi battery mission. That will be fixed in the next release V2.4.3