Do this upgrade at your own risk! You CAN damage your iMac or cause harm while dealing with exposed electronics.
Link to iFixit teardown guide, the 27" variant is nearly identical:
https://nl.ifixit.com/Guide/iMac+Intel+21.5-Inch+EMC+2428+GPU+Card+Replacement/5980
Link to Macrumors forum:
https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/
Link to Linux flash tool:
https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/post-28184981
I do not own any of the music. The rights go to the corresponding owners:
Magic by Allerlei von Nicolai | https://www.youtube.com/channel/UC_bcboyEwTxpEyM-fuCjLkA
Music promoted by https://www.free-stock-music.com
Creative Commons Attribution 3.0 Unported License
https://creativecommons.org/licenses/by/3.0/deed.en_US
Wandering by Numall Fix | https://soundcloud.com/numall-fix
Music promoted by https://www.free-stock-music.com
Creative Commons Attribution 3.0 Unported License
https://creativecommons.org/licenses/by/3.0/deed.en_US