As a reviewer, you'll get a notification that you have a pull request to review. You can also find open pull requests from the Pull requests tab on your Dashboard.
If you aren't satisfied with the proposed changes, you can decline a pull request. To decline a pull request, click the Decline button in the top left of the page.
If you'd like to explain why you're declining, you can add comments before or after. Once you decline a pull request, no one can reopen it. Whoever wants to merge the branch will have to open a new pull request from the same branch.