how do i create a new key?

Are you referring to the encryption key that is created when you encrypt your form?

Introducing-Encrypted-Forms-The-Ultimate-in-Online-Form-Security

You can create another key by disabling and then enabling the Encrypt Form Data option from Preferences of your Form Builder page under Advance Settings. 

The newly created key, however, can only be used for future submissions and not with the submissions that were on your form before it was created. 

If you have lost your old key, we would like to apologize, unfortunately, there is no way we can retrieve it as it is unique. To prevent this from happening I would suggest you always save a duplicate copy of the key and save it somewhere safe, like online storage websites(Dropbbox, GoogleDrive, One Drive etc...)