Per my research, request sign-off flow is a packed approval flow integrated between SharePoint and Flow. It is a built-in template flow for reuse in any list or document library. So, users just can use it and cannot find and edit request sign-off flow.
Noticed that a similar requirement has already been posted into the official feedback portal. I've added my vote on it, and I'll leave the link below in case you or other community users who see our discussion here would like to add vote or comment there as well. Hopefully this can be realized in the future.
In addition, you could go to approval flow templates to find the flow that meets your requirement.
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