If you're not able to edit your contact's email address, it means that we've sent them a message already. Once a message has been sent, the email address cannot be changed. To change the contact's email address, add them again as a new contact and enter in the correct email address for them. You can then remove their other email address where they were previously messaged.
If you see a bounced status for a contact, you can select the "Fix Email" option to clone the contact with a new email address. This way your stats remain the same.
To access the "Fix Email" option, click on the Bounced status for a dropdown.
A pop-up window will appear asking you for a new email address. This will then clone the contact with the new email address for you to restart messaging.