How to handle stuck updates? I've got a (current other than this latest update - edit macOS High Sierra 10.13.6 Update) OS X which has stopped updating.  It lists the update, goes through the install, says to reboot.  However, after rebooting the same update is in the list.  Looking at the history it has "installed" probably a dozen times now...  Obviously it's not working, but whatever is failing is happening silently.  No error messages.  Looking for debugging suggestions if not outright solutions such as "yeah that latest update is just bad".  :-)
Try the combo updater instead - https://support.apple.com/kb/DL1970 for a direct download link which you run like any app.

You can always Google for "10.x.y combo update" &amp; it will be the top hit.
It will apply all updates since 10.x.0 without needing any intermediate updates &amp; is often used to fix odd update glitches.

Failing that, then try reinstalling the OS from Recovery.
This will not affect existing apps, docs etc - though of course, as with any operation like this having a current backup is always wise.