There is no native undo feature for the Code Owners assignment in the app, e.g. when the wrong destination branch was chosen.
But there is a trick for fast removing reviewers of a PR in the PR edit dialog:
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Click on three dots(further actions) in PR view
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Click on Edit in opened menu
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Click behind last reviewer in Edit dialog, so that cursor is there.
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Press Delete until reviewers list is empty
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Press Save
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Trigger Code Owners in PR context menu "Code Owners: Trigger assignment"