Best site to read is
http://www.my-gti.com/ This will give you details of the firmware, what the different versions do, and also any known issues to watch out for. There is also a thread on firmware in the satnav section its quite long but worth a read, as some of it is a bit of a second language.
Tha basis of updating the RNS unit is to burn an image file (ISO, think of it as a picture of the content of a DVD or CD) onto a CD and then place this into your unit, the rest all just happens by itself.
A couple of things to be aware of whne messing around with your firmware, is the potential bricking unit issue (where the hardware in the unit isnt compatible with the new firmware) and power-drain issues with your cars CAN gateway.
The later Can gateway issue will depend on the age of your car, if the car (and therefore the gateway) was manufactured prior to when the RNS unit became available then you cant upgrade the firmware beyond 1100. You can replace the gateway (details on the site I linked at the top) and then upgrade to the latest firmware which will add a whole host of features as well as speeding the unit up.
Hope the above helps, once you have read up on the my-gti site and the firmware section, post back with any questions, one of the experts round here on the RNS is Morttygttdi sure he wouldnt mind you pming him once you have sussed out what you want to do to confirm details.
