Your primary email address acts as your unique username for Read. You can change your primary email address by going to your Account Settings > Profile & Account and clicking "Update":
However, you may see that your "Update" button is disabled with a warning like this:
In this case, to change the primary email address (the login email) for your account, you need to
- Click "Add Password"
- You'll then receive an email to create a password for your account.
- Once you've done this, you can update your primary email from the same Account Settings page.
After you update and click "Save Changes", you'll receive an email at the new address with a link to verify that it belongs to you. Your update will not be complete until you click that link.
Note that if you only care about using a different calendar for Read, you can do this without changing the primary email. Just go to Account Settings > Calendar, disconnect your current calendar and connect a different calendar instead.