You need to go to maintenance and first enable Apple updates (or iTunes updates - can’t remember which)
Then, you can go to regular ATV settings and click ‘Software Update’. This should begin the software download and install.
COUPLE OF POINTS: Upgrading to ATV3.0 will remove ATVFlash, so you will need to get the new update and create a new flash installer.
If you use an external hard drive for your main storage, change back to internal storage before enabling the update. I have had a lot of trouble with this - I followed the steps, but the ATV no longer identified my 180gb of data. I’ve had to reformat the entire disk and lost some HD shows in the process - grrr…
Stevehalluk - apologies if this has been covered elsewhere but if I do what you suggest will I wipe all the movie/TV content which I have previously copied over onto my Apple TV by ftp (ie. not through iTunes)? Cheers