During your registration, you will be given a master password recovery key. Make sure to copy it and store it safely. With a recovery key, you can reset your master password in case of loss.
That said, you will be directed to this window after you set a master password, save the key and agree with the terms. The recovery key is the only way to reset your master password.
You may check the recovery key guide, on how to reset it or use it to set a new master password.