Roles assigned to the tasks in the plan are listed:

For a listed role, click to select from a list of resources and roles or to process a search and find a suitable one. Before doing this, you can set the Date Restriction from and to dates so that only assignments during the specified period are affected by the mapping.

Once you click Map Roles to assign and save the resource or role, the previous role is removed from the list so that only unassigned ones appear.