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.
With that being said, you will be directed to this window after you set a master password, save the key and agree with the terms. The provided recovery key is the only way to reset your master password.
In case you have lost the saved code, but you are still able to log in, you can set up a new recovery key and it will act as a new code to reset your master password.
To reset it, go to Account in the menu and click Reset recovery key.