TYPO3 v9 has reached its end-of-life September 30th, 2021 and is not maintained by the community anymore. Looking for a stable version? Use the version switch on the top left.
You can order Extended Long Term Support (ELTS) 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.