Some OpenStack projects have their own upgrade instructions, for example Nova: https://docs.openstack.org/nova/lates... or Cinder: https://docs.openstack.org/cinder/lat....

The main challenge is updating the database, since database schemas often change from release to release. Rolling upgrades are now widely supported: You upgrade one server at a time, with the consequence that you run a mixed cloud during the upgrade interval.

Also read the release notes and look for summit videos that cover the subject.