Use Webupdater for a start. You can download this from Garmin. I've found it to be more reliable.
The instructions suggest that the unit will automatically restart when you unplug the unit. This is not so. Just switch it on the usual way (ie. press the power button) and once the unit has completed its restart, check in Tools -> Settings -> System -> About and look at the Software Version details. If it's up to 2.50 (as it is for my 3790T) then you're done. If it's an earlier version, plug it in again and run through the process again as the update may not have gone through. I had to go through the process twice for mine as it didn't take the first time.
There appear to be a bug in the software update for the 3790T try pulling the usb lead out without dismounting it, this is how I got the V2,5 to stick from my mac after a few attempts. (apparently this need to be done for various versions of windows too.)
It does not auto on and off, do that manually.
Now if only I could get my N American Maps to read on my European maped Nuvi before I visit the US!
Despite being loaded and unlocked the nuvi is "unable to unlock maps"
