I made a small change directly to a source module years ago - how do I back it out?

[Twincam]

I made a small change directly to one source module years ago, and now I want to back it out and create it as an addon. The system is correctly highlighting it as a failed file health check. Is there a simple way to revert it or Do i just need to download the source, find it and do it manually?
I made a small change directly to a source module years ago - how do I back it out?

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