You can use the comparison properties window to modify:
The name of the comparison.
The group labels used in all outcomes for that comparison.
To change the group labels for all outcomes for a comparison:
Open the outcome properties
Click the outcome and the click the Properties button in the outline toolbar.
Select Set Group Labels For All Outcomes.
Enter the labels you wish to use.
Note: Even if you only wish to modify one of the labels, you must fill in both in Group Label 1 and Group Label 2 . If you leave one blank, this would be applied to all outcomes.
Click OK.
The new labels are applied to all outcomes for that comparison.