Answer by VonC for Undo a merge by pull request?

Starting June 24th, 2014, you can try cancel a PR easily (See "Reverting a pull request") with:

Introducing the Revert Button

you can easily revert a pull request on GitHub by clicking Revert:

You'll be prompted to create a new pull request with the reverted changes:

It remains to be tested though if that revert uses -m or not (for reverting merges as well)

But Adil H Raza adds in the comments (Dec. 2019):

That is the expected behavior, it created a new branch and you can create a PR from this new branch to your master.
This way in the future you can un-revert the revert if need be, it is safest option and not directly changing your master.

Warning: Korayem points out in the comments that:

After a revert, let's say you did some further changes on Git branch and created a new PR from same source/destination branch.
You will find the PR showing only new changes, but nothing of what was there before reverting.

Korayem refers us to "Github: Changes ignored after revert (git cherry-pick, git rebase)" for more.

Also:

API for reverting a pull request (Jan. 2023)

A GraphQL mutation is now available for reverting a merged pull request: revertPullRequest.

Like the revert action on the pull request page in the web, calling this API creates a new pull request that reverses the changes made by the merged pull request.

Learn more about reverting a pull request.