For quick assignment changes, the Assign Resources dialog box is once again your best friend. After you add or remove resources, Project asks you what you're trying to do, and then modifies the appropriate values.
Here's how you add and remove resources with the Assign Resources dialog box:
On the Standard toolbar, click Assign Resources.
The Assign Resources dialog box opens.
In the Gantt Chart view or any other task-oriented view, select the task you want to modify.
The resources already assigned to the task appear at the top of the list, preceded by a checkmark.
To assign another resource to the task, in the Resource Name column, click the name of the resource, and then click Assign.
To remove an existing resource from the task, do the same thing but click Remove. The assigned checkmark disappears, and the resource takes its place back in the alphabetical list of unassigned resources.
To change the units for the new assignment, in the Units cell, type the percentage.
If you remove a resource, Project automatically clears the resource's Units cell.
Repeat steps 3 and 4 to add or remove other resources.
You can add or remove several resources before you close the dialog box or select another task.
When you're done, either select another task in the Gantt Chart table or close the Assign Resources dialog box.
In the Gantt Chart table, a green feedback triangle may appear in the Task Name cell's upper-left corner, advising you that Project needs some help ...