Thread:Extension talk:WantedPagesFromNS/Taking over an old extension/reply

See Manual:Developing extensions. Since this extension isn't in any official Wikimedia repo, there's no real process; you can just make a new version, publish it somewhere and link to it from the extension page.

If you want to move the code to an official Wikimedia git repo and become the new project owner, you can create a git account and request a new repo for it. That's a bit more work, but makes it less likely that the extension will be neglected in the future.

Or you might want to consider writing a patch to integrate this functionality into MediaWiki core (as part of Special:WantedPages) and submit it to bugzilla. That might actually be the best solution.