Could you further explain? Are you trying to upgrade to iOS 6.1.3? If so, have you connected the phone to iTunes and pressed the Restore (or Upgrade) button? If yes - what is the error you are getting?
Or, are you trying to do the over-the-air (OTA) update via your Settings app? If that is the case, OTA doesn't work on jail broken devices; you'll have to use iTunes to upgrade.
Just be aware that when you do get the 4S upgraded to iOS 6.1.3 - there is no jail break available for that iOS (and none will be made available). You will have a stock iPhone with no way to downgrade to a different iOS.
Marilyn