To change assigned resources for a task that is multiple-resource :
open the Schedule editing dialog for the task. Resources may be added and removed from the Resources table on the lower left, shown below
to add a new resource click on the empty cell in the first column of the table.
a resource selection dialog appears where you can select the resources you wish to add.
click OK to close the dialog and the selected resources are added to the table.
to delete a resource from the table, select the resource's row and click the Delete button.
Ensure that the Assign work to all resources button on the upper right is selected, otherwise the task will be interpreted as a single-resource task.