Nokia is showing a lot of laziness in release substantial updates, but folks at windowsmania have released Amber (GDR2) firmware. In fact, we also have the instructions on flashing these stock ROMs. However, keep in mind this is like installting fresh ROM on your Lumia 920 / 820, which means it will wipe all of your data.
So, you should make a backup of anything and everything you have on your windows phone before you hit the flash button. Find the download instructions at these links. For 920 [link]; for 820 [link]. You can flash the ROM, irrespective of color of your Lumia.
Update – We now know how to flash it.
Download the files from aforementioned links and follow these instructions.
1. Download and install latest version of Nokia Care suit.
2. Download the driver from here – http://d-h.st/Mc3
3. Put the downloaded files in this folder – C:\Program Data\Nokia\Packages\Products\rm-821. Create any folder, if it is not there.
4. Go to File > Open Product and select RM-821
5. Select Product Support Tool as shown above.
6. From Programming menu, select “Recovery” and flash your firmware from there.