Boot into A/B slots

Today I messed with my /etc/fstab file, so the OS did not boot anymore. Because there is no display port available in order to work with a live USB boot system, I had to fix the SSD in another machine.

There I saw there was a perfectly fine OS on a second partition I could have used to boot from. So the question is for the next time: how can I boot into the other slot/partition in case the current slot fails to boot? Thanks!