Encryption and Decryption of User ids and Passwords

I have download the latest FBA pack, followed the steps provided for installation and started using it. Can you please share the information on how the UserName and Password are encrypted before saved in the [aspnet_Membership] table. I need to decrypt them, so that I can use them for changing password from external applications
How the password is stored is actually set by your membership provider settings in your .config file(s). There's a PasswordFormat parameter:
Clear: passwords stored in clear text
Encrypted: Passwords are encrypted and decrypted using your machine key.
Hashed (recommended): Passwords are hashed and the original password is not recoverable.

If you're changing the password from external applications - you can use the membership provider API to make the changes:

https://msdn.microsoft.com/en-us/library/system.web.security.membershipprovider(v=vs.110).aspx