Once the launch form of a workflow has been submitted, you can change which user is assigned to review and approve the document on the Review page by clicking the three small dots to the right of the current approver’s name and then selecting “Assign to…”.
Note: If you're unable to see the button that means you're not in the same group as the approver.
After clicking Assign to... button the approver's name will become a dropdown menu and you can select a new user from the list. Only individuals assigned to that group in your company settings will appear in the dropdown.
For more information on groups, please see the Groups and Permissions article.