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 in Scheduled. The moment Stage 1 is queued (i.e campaign has passed the "Scheduled" status), you won't be able to alter settings of a campaign.
However you can edit the content (i.e. subject and body of follow-up emails) of the subsequent stage before they are triggered but the settings froze in stage 1 can't be altered.