In Templates, find the template you'd like to make updates to and select Edit from the more options icon to the right of each template name.
Make any updates you'd like to the template subject or body, click Save Changes and you're all set!
Will editing my saved template update my current campaigns that use that template?
No - edits to your saved template will only affect the saved template and future campaigns you create with your updated template.
Campaigns that were previously created or submitted with your saved template will remain as they were when the template was applied at that time (i.e. changes you make now will not automatically apply). You can edit any touch for a current campaign like this.