To change the assigned resource for an entire subtree of single-resource tasks
select the parent project of the subtree, and double-click in the parent project's cell in the Assigned To column of the Attribute table view
the assignment dialog below appears
select the resource you want from the drop-down list
check the projects that you want the resource assigned to
click the Set property for selected projects button to assign the resource to the checked projects
click OK to save the changes.