Well, two ways. You can just edit it in the Preview window, same way as when not-live. People will see the alterations you're making in real-time as stuff gets moved/resized/added/etc.
You can also click the Studio Mode button in the bottom-right, which will give you two windows... the right-hand one is your active outgoing scene, and the left you can freely edit without anyone seeing the changes made until you hit the Transition button in the middle.
I tend to swap back and forth between standard and studio mode, using studio when I need to add a new source or change something (like my Game Capture target) behind-the-scenes before swapping over. Standard mode is easier to see though, when you are just switching between scenes and aren't making any changes.