I head to the maker's support web page for the product.
Example? http://store.archos.com/docs/Android%204%200%20Update%20Fact%20Sheet.pdf
Never had to go elsewhere. Now with that out of the way there are other ways around this but YOUR TABLET, YOUR CHOICE, YOUR RISK!
Example? -> http://forum.xda-developers.com/showthread.php?t=1738505
Bob
Google has said the option on if you can upgrade Android is with the tablet manufacturer. Does anyone knows if Archos allows this? OR, can a 512 MB RAM/ ARM cortex A8 1.2 GHz tablet upgrade above Android 3.2 to begin with?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic