An update has three stages. WordPress downloads the package, extracts it and replaces the old files. If any of them is interrupted, the site is left in a halfway state where some files are new and some are old. That is where every symptom in this guide comes from.
If the site says it is briefly unavailable, open it first. The steps are in our guide on WordPress stuck in maintenance mode. Only once the site responds can you see the update state reliably.
WordPress sets a lock in the database while updating. If the update breaks off, the lock stays. It expires on its own in about fifteen minutes, so the quickest action is to wait. If you would rather not, the lock can be removed from the database. It is in the table whose name ends in _options and the row is called core_updater.lock. See our guide on managing the database.
Open the updates screen in the admin. It shows which plugin or theme is still on the old version. Run the updates again one at a time rather than several at once. That way you see immediately which one is problematic, if any.
Then this is not chance. The usual causes are a time limit, a memory limit and running out of disk space. A large plugin can fail simply because extraction runs out of time or memory. Running out of memory is covered in our guide on memory running out.
A plugin can also be updated over a file connection. Download the new version, back up the old folder, delete the old folder from wp-content/plugins and put the new one in its place. The settings are in the database and do not disappear. The same works for a theme. Making the connection is covered in our guide to FTP and SFTP.
That is a different matter from a failed update. See our guide on a critical error on the site, which shows what crashed and why.
Do not restart the update the same second the previous one broke off. Two simultaneous updates write the same files and the result is a mess. Take a backup before updating, see our guide on backing up a site.
We finish the interrupted update, check that the halfway state did not break anything and fix the reason it stopped. After that updates go through without manual work.
