Have a JTAG or RGH modded 360 and need to update your Dashboard?  This  is the easiest way to go about doing it yourself AND updates your avatar  files so you don't have a faceless gray blob when you're done.  This  will update you to Dashboard 17559 but can be used for any version you  want to update to.  If a new dash version is released in the future  (unlikely) I will update these links to the new dash version.
https://www.youtube.com/watch?v=ezy1soP7wJA