Thank you for reaching out to the Community. That is a great question! You can change your default email address in the My Account section located at the top right of your profile. You can find the steps by clicking here.
For email campaigns that are already created, you will need to manually change the email address in the header. You can click here to view these steps.
In the meantime, if you have any further questions please feel free to reach out to us here. We are always happy to assist you in any way we can.