Workflow Managers can view and edit many aspects of their workflow including all tasks and assignments. By default, the person who created the workflow will also be the first Workflow Manager on the workflow. See this article for more information on what they have access to see and modify. 

Navigate to the workflow and scroll down so you can see the Workflow Managers section on the bottom-right side of the screen. Click on the pencil next to the title of the field.

A pop up will appear. Use the search box to find the person you would like to add as an manager for the workflow or add them as a new profile and click Save to finish. Note: Guest users can't be added as Workflow Managers. They will need at least the Limited Read People permission to be able to handle reassignment requests.

To remove a Workflow Manager, navigate to the workflow which contains the manager you would like to remove. Scroll to the Workflow Managers section on the bottom-right side of the screen. Click on the pencil next to the title of the field, and then click on the red X button to the right of the person's name.

Note: You must always have at least one Workflow Manager. Add a second one before deleting the first, when needed.