Can I edit campaign after it is scheduled?
Yes, you can edit campaign till the campaign is in Scheduled Status.
You can edit the content and modify the settings till your Stage 1 is scheduled. The moment Stage 1 is queued (i.e campaign has passed the "Scheduled" status**), you won't be able to alter your campaign.
However, you can edit the content (i.e. subject and body of follow-up emails) of the subsequent stage before they are triggered.