To edit Boolean objects, you should click on one of the operand objects in the scene tree and edit it via the property window or in the 2D/3D view.

In the following example a cuboid defined as round was subtracted from a rectangular cuboid. 

If you want to change the cylinder afterwards, you should select this in the scene tree. 

If a sub-object of a Boolean group is selected, then the operand objects are shown in addition to the generated object (in this case the cuboid with the hole). 

The selected object is colored orange and the non-selected ones cyan.

Editing objects inside a Boolean group.

1. The Boolean object generated from a rectangular and a round cuboid.

2. Select the round cuboid. The cuboid selected is orange and ready for editing.

3. Change the radius of the round cuboid.

4. Move the round cuboid.

5. Deselect the object.

The function of interactive duplication keeping the " CRTL"  key pressed can also be applied inside Boolean groups.

In the example, additional holes could be punched out of the rectangular cuboid by moving them while keeping the " CTRL"  key pressed.