If you want to change your MyFarys username, you need to delete your existing account and re-register.
Go to the Customer data page. Please note, it can take up to 1 working day for your old account to be deleted. Only your username and password will be deleted. Other settings such as digital billing, paying by direct debit... will obviously be kept for your customer number.
Only after your old account has been permanently deleted, you can register on MyFarys with your new e-mail address.