To change the email template for an individual campaign, go to the Campaigns tab and select the campaign you'd like to modify.
Once you've opened up the individual campaign, go to the Emails tab:
Next, open up any one of the individual emails you've already created in the campaign. It doesn't matter which of the emails you select because campaigns automatically update all other emails in the campaign with the same template. There is currently no way to use multiple email templates for an individual campaign.
Once you've opened up one of the emails in the campaign, go ahead and navigate to the Preview tab. Click Change for all emails in this campaign, located directly underneath the email footer.
When the Template Gallery appears, select the email template you'd like to use for the campaign.
Click the Save button once you've made your selection.
The campaign emails will now render with the new email template you've selected.