Check if the customer is trying to change the email address of their own order by asking for a proof of payment, email address and/or full name.
Search for the corresponding order in the backoffice.
Click 'Edit' in the upper right corner (next to 'Client').
Change the email address on the left and click 'Save'.
If you receive a notification saying the account already exists, search for the NEW email address in the column on the right. Select this email address from the dropdown and click 'Save new'.
The email address is now changed. Click 'Resend tickets' in the lower left corner of the screen to resend the tickets to the correct email address. This will be done within several minutes.