How do I force macOS to update? When I try to open Xcode I get the message that I'm not running a new enough version of macOS. However, the software updater menu says I'm up-to-date.
You should be able to download an update manually.
Generally speaking Apple releases what they call combo updates that will update all previous versions of an OS release to the (current) latest version.
You can search out "Mac OS 14 combo update" and you will end up here:
With the latest revision of macOS Mojave 14.5.
This also works with previous versions of macOS. Combo updates are also a good way to fix a "misbehaving" OS installation.