The Application password should not be changed as you will loose the ability to read the data later. You can use the license of the firewall to encrypt and decrypt data.
You can use the IFireWall.License.Encrypt end IFireWall.License.Decrypt to protect any data using your license and the application password.
options.Cypher.ApplicationPassword = "mydomain".AsShaHash(HashMethod.SHA1, 8);
Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2