Multiple Resources on an Activity

Lets take an Activity with Multiple resources

Resource     Budget Units     Actual Units     Remaining Units     At Comp Units

Carp               500                         0                         500                    500

Labr                250                         0                         250                    250

Iwrk                  75                         0                            75                     75

when you apply the units pct comp.  it takes the total hours and use it to calculate the individual resources.  How can I change the units pct comp for each resource independently through a global change or something