From Joomla! Documentation

Check out as much of the functionality on your test site as possible. If there are any problems that will affect the way that you will need to handle the live upgrade then make a written note of them for later.

If you find errors you might check out the following possibilities:

  • Ensure that all of the files were properly transferred. There have been verified reports of some FTP clients not properly transferring files across to a server without notifying the user of such a problem. One possible cause of this problem is that under certain circumstances the web server locks the files it is using and the FTP-server can't update those files. If this is the case you might try taking the site offline shortly during the FTP transfer.