You can't overwrite the email address you use to login. If you want to change it, you will have to do so by adding a new user.
In the 'User' tab, click on 'Show all users' at the bottom. Now click on the button '+Add' in the upper right corner. Enter the new email address you want to use to login and indicate the rights you want to assign to this address. Confirm at the bottom with 'Save'.
You will receive an email with your password at this new address. Now log out with the button at the top right and log in again with your new email address and the password you received by mail. Go back to your user overview and click on the icon under 'Actions' next to the address you want to delete. At the top, turn off the switch next to 'Active' to deactivate this address. If you do not do this, there will be an extra charge of € 5/month for this extra user.