Assuming you're already running a customized Greenlight Docker container...
https://docs.bigbluebutton.org/greenlight/gl-customize.htmlInside the greenlight git directory, go to app/views/shared/_header.html.erb. Search for the string
http://docs.bigbluebutton.org/install/greenlight-v2.html, and replace the link with your own. Rebuild and restart greenlight, and it should change for you.
If you do update Greenlight to the current version, you'll see a new option for reporting an issue in that dropdown. This link is set in your .env file, so you don't need to rebuild the docker container, just change the .env and restart