If you want to change your email address, log into your account at my.mailmergic.com. Open the user menu in the upper right corner and click on « Account« . You will be redirected to your account page.
Click on the Edit button in the « Email address » area. Enter your new email address and click on « Save« :

You will now be asked to enter your current password in order to prevent unauthorized changes to the email address:

After you have successfully entered your password, your new email address will be saved, which you can use for login from now on.
To be on the safe side, a confirmation email will be sent to your old email address that your email address has been changed.