Select all pads that you want to change by holding shift and individual selecting or group selecting them.
I suggest disabling the selection of all other objects using the select toolbar to avoid accidentally selecting any other type of object as that would prevent setting the drill size.
Right click any of the selected pads, click properties and change the drill size in the pad tab as you would if you were changing them individually.
Be careful with your selections as you can only change them to a single drill size at a time
