How do I use "Publish as" to publish to a game hosted by someone else?

I think the title is self-explanatory, but I’ve been working on an update for somebody else’s game. The update was made in a separate place owned by me, and the owner of the game itself is currently unable to publish it to his own game. When I try to use “Save As” or “Publish As”, the only options Roblox gives me are my own games and games owned by my groups. How can I get around this?