Do I have the latest firmware update for my Time Capsule? How can I force an upgrade of the firmware?
If AirPort Utility doesn't prompt you that an update is available, then you have the latest version installed on the Time Capsule.


  How do I know when an update is available?
  
  
  AirPort Utility automatically checks for updates once a week (even if it's not open), then notifies you if an update is available.
  When you open AirPort Utility, it will check for updates and display a badge by the name of any AirPort device in your network that has an available firmware update.
  


Source: http://support.apple.com/kb/HT1218