You will need the Full Write Workflow permission to perform these actions.
Workflow admins can view and edit many aspects of their workflow including all processes and tasks. By default, the person who created the workflow will also be the first admin on the workflow. See this article for more information on what they have access to see and modify.
Adding an Admin
Navigate to the workflow and select "Add admin" from the "Actions" menu in the upper right.
A pop up will appear. Use the search box to find the person you would like to add as an admin to the workflow and click on "Add" to finish.
Removing an Admin
To remove an admin, navigate to the workflow which contains the admin you would like to remove. Then click on the "X" button to the right of the person under the "Workflow Admins" box on the right side.
Note: you must always have at least one admin on a workflow. Therefore, you will not see the "X" button unless there are at least two.
A confirmation message will appear. Click on "Yes" to finish.