Hash Password Decryption

Does somebody know which method I can use to decrypt an HashPassword cryptd with

FormsAuthentication.HashPasswordForStoringInConfigFile(MyPassword. "SHA1")

I know obviously the security issues but if I am an Admin and I want to manage some Users password, how I am suppose to do ?

UPDATE:

Hi,

here is a code example for encrypting and decrypting using SHA1

http://www.obviex.com/Samples/Encryption.aspx

Martin Spedding 

Thanks Martin ;-)