Here we distinguish two cases. Case 1: You want to change the email to which your invoices are sent. Case 2: You want to change the email to log in to your account.
Case 1:
Log in to your GraphHopper account. On the main page you will see a button called Invoice and Payment Data.
This button opens a mask where you can change your personal data like email and address.
Case 2:
Log in to your GraphHopper account and just click on the email in the upper right corner as it is illustrated in the figure below. Then you can change your email.
Please note that this also automatically changes the email to which your invoices are sent, i.e. if the invoices are to be sent to a different email, you must go back to Case 1.