When will the new iso based on 13.1 be released?
Which iso? The one for FreeBSD (which may or may not be an .iso file depending on the architecture you are running it) or the one for NomadBSD (which is definitely not an .iso but an .img file)?
Just do an upgrade. I did it yesterday without issues.
# freebsd-update -r FreeBSD 13.1-RELEASE upgrade # shutdown -r now # freebsd-update install
Follow the instructions provided with the upgrade. If you have custom configurations, such as in /etc/groups, it will have you merge those files manually during the upgrade. Just follow the instructions provided during the upgrade.
For more details on upgrading to FreeBSD 13.1 look in this thread.