We know. It happens.
You just need to enter the email address you use for your Zalando account and click send. You'll receive an email with a password reset link in it (please note: this email may take a couple of hours to reach you and could appear in your spam / junk folder).
Click on the link in the email and you will be prompted to enter a new password.
You can then use your new password to login to your Zalando account.