TYPO3 v10 has reached end-of-life as of April 30th 2023 and is no longer being maintained. Use the version switcher on the top left of this page to select documentation for a supported version of TYPO3.
Need more time before upgrading? You can purchase Extended Long Term Support (ELTS) for TYPO3 v10 here: TYPO3 ELTS.
Make a backup first! If things go wrong, you can at least go back to the old version. You need a backup of
all files of your TYPO3 installation (by using FTP, SCP, rsync, or any other method)
the database (by exporting the database to an SQL file)
Also you may prefer to upgrade a copy of your site first, if there have been a lot of changes and some of them might interfere with functions of your site. See the changelog to check that.
For more detailed information about TYPO3 backups see Backup Strategy in TYPO3 Explained.