Any GitHub wiki can be cloned by appending wiki.git to the repo url, so the clone url for
the repo https://myorg/myrepo/ is: [email protected]/myorg/myrepo.wiki.git (for ssh) or https://github.com/my/myrepo.wiki.git (for https).
You make edits, and commit and push your changes, like any normal repo.
You need to clone the wiki repo and edit it on your system.