Can you edit without launching?

Can you edit a campaign without also launching it? If so, how?

Nope.

You can talk to your players not to get in until you say so. Or launching it at a different time and tweaking it.

Players can join even if the gm isn't there.  I do it now & again to tweak my macro's.

Most GMs I know leave players on a neutral page that they can play on in case they log in for macro tweaking ect. Where any running around or drawings they do on the map would not do any harm

Why would launching a campaign be an issue? I mean why do you want to edit the campaign without launching it?

One thing I find useful is I can launch in roll20, make changes etc, then launch in hangout & the changes are there. Probably not helpful unless you play in hangout but... :-) Edit: and as Terratani says, I leave my players on a landing page (at the moment the previous encounter, though as we finished in the middle of combat last week, I think I'll probably change that) as they come in, then move them to wherever I've got planned when they all get there.

Oh... do you mean work on a campaign without launching a google hangout ? If so... you can definitely do that. Just click the 'Launch Campaign' instead of 'Launch in Google+ Hangout'.