Your email address defines your Birdee ID. By changing it, you are also changing your login.
To change your email address, the procedure is as follows:
Log into your customer area
Click in your user tab, on "My information"
Go to the sub-section "Access"
Click on the "Edit" button next to your email address
Enter your email address
Click on "Send me a validation email"
Enter your password to authenticate the changes
Click on the "Validate" button inside the e-mail sent to you
Click on the "Connect" button on the confirmation page
Log in with your new email address