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.