If an update is available but not possible to perform, OctoPrint will usually show you a brief error message in the UI, with more information logged in octoprint.log.
What follows are some next steps to do in case of any issues like that.
OctoPrint says my Python environment is outdated
Please see this dedicated FAQ entry:
OctoPrint says something about insufficient disk space
If you do not have enough space to update, you'll have to make space first. Check if there are timelapses you no longer need that you can delete (they take up a lot of space). Also check if you can clean up some of your GCODE files.
OctoPrint is installed on a Raspberry Pi and says something about a throttled system
Updates are disabled while the RPi is actively throttled either due to undervoltage or overheating. Over the years we've seen way too many corrupted installs created when attempting to update under such circumstances, which is this is now disabled by default.
See this FAQ entry for more information on how to proceed:
OctoPrint is installed under Windows and cannot be updated
If you have installed the OctoPrint server under Windows, that is sadly to be expected. Windows has some aggressive file locking built in that prevents the modification of files that are currently open or being executed. That means OctoPrint cannot update itself when running under Windows, as it would have to replace the very files that are currently running.
To update OctoPrint under Windows, you'll have to stop the server, update OctoPrint manually from the command line (with your OctoPrint venv active, run pip install -U octoprint), then restart the server.
For installations using the Windows Installer by @jneilliii there is a start menu entry that can be used to perform this process named Upgrade OctoPrint. Make sure you are not printing when using this option.
OctoPrint 1.10.0 and later also recognizes this situation and doesn't even offer updating OctoPrint through itself under these circumstances. Prior versions will attempt to update themselves but the update will fail.
A third party plugin cannot be updated
If it's only one specific third party plugin that cannot be updated (and not all of them), please should seek help in the plugin's issue tracker. There might already be a ticket about this issue, otherwise create a new one. In any case, provide a systeminfo bundle so the plugin author has the most important logs available right away, and stand by to provide further information to them as well.
Anything else?
If that isn't the issue, then please submit a support ticket in Get Help. Fill in the whole template and make sure to share your system info bundle, we won't be able to help you otherwise!