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Simply clink the "Fork Settings" button and confirm that your really want to delete your fork.
Multiple Forks. As a general rule it is important not to mix functionally unrelated code into the same pull-request. Rather, you should consider using multiple forks. If the current fork is tied up waiting for a PR to be closed and you want to submit an unrelated PR, just create a new fork for the second PR.
REVISIT: By emailing a patch: Describe how to make the relevant patch(es) (git format-patch) and send them to dev@nuttx.apache.org.
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