You can edit a campaign that has been scheduled, but not yet delivered. On your dashboard, you'll see a list of all scheduled campaigns on the right hand side under 'Scheduled Campaigns'. Find the scheduled campaign you'd like to edit. From its 'More actions' menu, choose 'Unschedule'. It will then be saved as a draft. To open it, go to 'Campaign' in the primary menu and then select 'Open saved draft' from under the 'Reuse' heading. Find your saved draft here and click 'Open'. From the 'Campaign Summary' page, you can make any edits you like before re-targeting and re-scheduling it for delivery.
Please note that drafts will be saved for up to 30 days.