To edit who holds various Positions within your organization, navigate to your organization's Action Center and click on the Roster tool.
If you first need to create a new Position, visit this article to read about that process. Note that you will need to hold the correct level of access to manage the Roster in order to make these changes.
On the Roster page, find the user for whom you would like to add, change, or remove a Position. Click the edit button under Positions to the far right of that user's name. A list of all available Positions will populate. Click the check box next to the position you would like to add or remove from that user, and then select Save. The user's permissions within the organization will be updated based on the Positions they hold.