Oh, well in that case you should proceed directly to the Keyframe Palette (Window>Show Keyframe Palette) and make the change there. :)
Actually we added the Keyframe Palette to the product for RED 4 and must have missed that in the documentation. Sorry about that. The advantage to having in a palette is that it is a live, non-modal window and you can change it on the fly without having to go to the preferences panel.