Look at the editpad boundary command (Tools > Pad > Boundary)
The Pad boundary option is only avaliable in the Allegro license options. It's not there in OrCAD PCB but what you can do is use Tools - Padstack - Modify Design Padstack, then select the Pad. In the Options fold out menu make sure you select Instance so that this edit is only for this pad only (it will rename the padname to padname_1) then click on Edit and change the pad size accordingly in Pad Designer. Once complete use File - Update to Design and Exit and the pad will be replaced.