Here are the different ways to change the resource assigned to work.
To reassign all segments
Select the reassign resource option.
Select a resource to assign the work to or use check availability to browse for alternative resources based on their availability.
To reassign a single segment
Click an assignment bar.
Select re-assign.
Select the resource selector.
Select a resource to re-assign the work to (you can also select an alternate project and sprint to move the work to).
Select re-assign.