Once you've forked the RetroPie-Setup repository (as you've already done), create a branch in it and modify the file you want. Then you can submit a PR from the browser, based on your modified branch - https://docs.github.com/en/github/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork.

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