The release of Ubuntu 23.10 has not actually completed.
You can refer to this notice for clues
https://discourse.ubuntu.com/t/announcement-ubuntu-desktop-23-10-release-image-is-being-updated-to-resolve-a-malicious-translation-incident/39365
The respins of ISOs were done, and are currently in QA (Quality Assurance) testing, the status you can view by looking at
https://iso.qa.ubuntu.com/qatracker/milestones/449/builds
Currently the respun ISOs are not marked "(ready)" you'll note.
After the ISOs are released, and the release cycle of Ubuntu 23.10 completes; the Ubuntu Release team will explore the option of opening the upgrade cycle
The Ubuntu release upgrader tools check this file for status of upgrade
which is simple text (thus easily read) where you'll note no Ubuntu 23.10 currently lists; that is why the upgrade process is not seen.
You can force release-upgrade via use of -d as per instructions provided on Ubuntu 23.10 release notes & announcements, ie. read this link
https://help.ubuntu.com/community/ManticUpgrades
The -d works as it will force use of the following file, where mantic or 23.10 is seen
For best results; I suggest waiting though. I've seen 9+ emails hitting my inbox on the weekend representing work being performed on the release-upgrade bugs, the upgrade path will be open when the current work is completed.
Ubuntu 23.10 currently hasn't fully released; but the release is the initial step for new installs. After release completes, the Ubuntu Release team start evaluating the release-upgrade steps, opening the taps when its deemed ready; which has not occurred yet.
