On our current email sending strategy once you create a new campaign Symbol generates the schedule of emails that will be delivered during the period of the campaign, if the campaign has already started you'll not be able to edit this because it could alter the schedule of the campaign and some emails couldn't be delivered.

We want to provide you the best user experience and due to the complexity of our Email Sending Strategy you cannot edit running campaigns, if you would like to know more information about the best practices to create effective campaigns read our documentation about campaigns.