You will need to have the Full Write Workflow permission or be an admin on the associated workflow.
Process managers can see and edit most aspects of their process including all tasks. See this article for details on what they can do.
Adding a Manager
Navigate to the workflow containing the process and then click on the process in the listing. Next, select "Add manager" from the "Actions" menu in the top right area.
A pop up will appear. Use the search box to find the person you would like to add as a manager on the process. When done, click on "Add" to finish.
Removing a Manager
To remove a manager, navigate to the process and click on the "X" button to the right of the person you want to remove under the "Process Managers" box on the right.
A confirmation message will appear. Click on "Yes" to finish.