Looks like Nokia Amber firmware update is now available for the Nokia Lumia 820 and Nokia Lumia 920. According to WindowsMania.pl they got their hands on unreleased Nokia Amber firmware update.
Apart from firmware update, there is a full set of instructions listed on how to flash stock ROMS. You can find out the full details from here for Nokia Lumia 820 and Nokia Lumia 920
[Via] WindowsMania.pl