Once a campaign has been processed by our system, it can no longer be edited. It can only be paused, resumed or deleted. The reason is that it takes a decent amount of time to import a SugarCRM Target List(s), process the list(s) against opt-outs, invalid addresses, suppression list and removing duplicates. If you were to make a change to the Target List, for example, then that entire process would have to be started again - which could potentially delay your campaign an hour or more (depending on the Target List size).
Always be sure to test, test, test and test again before you schedule your campaign. You'll only be able to make changes up until 15 minutes prior to the scheduled deployment. After that, you'll only be able to Pause the campaign, Resume the campaign (if it was paused) and Cancel/Delete the campaign.