Prior to performing either a Ceph upgrade or an operating system upgrade all charms should be upgraded to their latest stable revision, which is the default behaviour of the upgrade-charm command (this can potentially be a multi-revision upgrade). See Upgrade target revisions for more information.
The Ceph charms have been tested for single-step charm upgrades only (N+1).
The Juju command to use for upgrading charms is upgrade-charm. For extra guidance see Upgrading applications in the Juju documentation.
In terms of the upgrade order, begin with ceph-mon, after which the remaining charms can be upgraded in any order:
juju upgrade-charm ceph-mon
Verify the success of an upgrade before moving on to the next charm. Do this by inspecting the juju status command output.