It wouldn’t automatically update container contents.
This looks simple enough if you are brave enough !.. nothing relative to Chrome OS … just a standard set of Debian upgrade scripts…Download it.
https://gitlab.com/ppulfer-random-stuff/debian-buster-in-crostini/blob/master/upgrade_container.sh
From within Debian, open a terminal and type: cd /home/felicity/Downloads [enter] (or if not Downloads, then correct folder)
dir [enter] - this will list the contents of the directory - make sure it is there.
Then type: chmod +x upgrade_container.sh [enter]
Then type ./upgrade_container.sh [enter]
Grab a coffee and cake and wait for the upgrade to complete.