The Encryption key change button allows you to change the key, with which user data is encrypted. Doing so will result in irreversible loss of all data stored on the server disk, which were sent as backup and synchronized with the Briefcase, from all user devices. While changing the encryption key, the user is asked to provide his account password, then he has to confirm the notification.
If you do not know how to go to the above window, check the instructions for application settings.