The email address you use in Contentful is used both for logging in and to receive notification emails about changes in your space.
To change your email:
Open the Contentful web app and go to User profile.
Click Account Settings.
Click the pencil icon next to Account.
Enter a new email address, then your current password to confirm and save changes.
You will receive an email from Contentful at the new email address. Follow the instructions in the email to confirm your new email address.