Replace resources assigned to activities

Choose Project, Activities.
Select the activity for which you want to replace a resource.
Display Activity Details, then click the Resources tab.

If resource security is enabled you can only view resources you have access to.
Select the resource you want to replace.
Click Add Resource.
Click the Replace button in the Assign Resources dialog box.
Select the new resource with which you want to replace the existing resource, then click the Assign button.
Click the Close button.

Notes

You cannot replace a resource that has actual dates assigned to it.
In the User Preferences, Calculations tab, you can choose to always use the units/time and overtime factor of the current assignment or of the new resource replacing the existing assignment; or, you can choose to be prompted to select which units/time and overtime factor you want to use each time you replace a resource on an existing activity assignment.
When you replace a resource on an assignment that has manual future period values, the manual values are preserved with the new assignment.