If a pull request has changed since it was reviewed and the person who requested changes isn't available to give an approving review, repository administrators or people with write access can dismiss a review. This changes the status of the review to a review comment. When you dismiss a review, you must add a comment explaining why you dismissed it. Your comment will be added to the pull request conversation.
You can find a pull request where you or a team you're a member of is requested for review with the search qualifier
team-review-requested:[TEAMNAME]. For more information, see "Searching issues and pull requests."
Under your repository name, click Pull requests.
In the list of pull requests, click the pull request you'd like to review.
On the "Conversation" tab, next to the summary of reviews, click .
Next. to the review you'd like to dismiss, select the dropdown menu, then click Dismiss review.
Type your reason for dismissing the review, then click Dismiss review.