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.