Once I selected a remote, How do I change it to another one?

Selected origin remote instead of upstream so I landed here too. I tried enabling and disabling the package but that didn't work.
There's some missing UI here to get back to the remote selection state (/cc @simurai for thoughts).
In the meantime, the current remote is stored in your repository's git configuration, so you can edit that directly:
git config --unset atomGithub.currentRemote
For now could be similar to a pinned PR:

But if, then only short term. Some sort of GitHub browser/filter, like https://github.com/atom/github/pull/714#issuecomment-298145826 is still the better approach.
Most helpful comment
There's some missing UI here to get back to the remote selection state (/cc @simurai for thoughts).
In the meantime, the current remote is stored in your repository's git configuration, so you can edit that directly: