Per the docs on the Ceph charm’s charmstore page, the charm is deprecated and is not receiving updates.
The OpenStack charmers team maintains versions of the charm with more specific components of ceph. For example, ceph-mon and ceph-osd. Those versions are updated on cadence with the rest of the OpenStack charmers releases.
As @pmatulis’s graphic says, you’ll have to update the Ubuntu Cloud Archive if you want to upgrade from Ceph Luminious version 12.2.x assuming you’re on Ubuntu 18.04.
You can change the source charm config on both the ceph-mon and ceph-osd charm in order for the charm to upgrade Ceph.
Do note that upgrading is not always straight forward so please read related release notes and test first.
For charm specific info, I recommend checking out the release notes and the upgrading appendix for the charms