how to decrypt password?

You could create a class that use TripleDESCryptoServiceProvider similar with my class bellow, and use it.

/// <summary>
/// The basic class for encrypt/decrypt data.
/// </summary>
/// <remarks>
/// It uses a TripleDESCryptoServiceProvider object.
/// </remarks>
public class Des3 : IDisposable
{
    /// <summary>
    /// The used DES crypto service provider.
    /// </summary>
    private TripleDESCryptoServiceProvider _des = new TripleDESCryptoServiceProvider();

    /// <summary>
    /// Initializes a new instance of the Des3 class.
    /// </summary>
    public Des3()
    {
        //
        // Sets the data for the encryption algorithm.
        //
        string password = "YourSecreetKeyMustBeHere!!!"; //You should change this value!
        MD5CryptoServiceProvider hashmd5 = new MD5CryptoServiceProvider();
        byte[] passwordHash = hashmd5.ComputeHash(ASCIIEncoding.ASCII.GetBytes(password));
        hashmd5.Clear();
        //
        // The key is the secret password hash.
        //
        _des.Key = passwordHash;
        _des.Mode = CipherMode.ECB;
    }

    /// <summary>
    /// Used as destructor.
    /// </summary>
    public void Dispose()
    {
        if (_des != null)
        {
            _des.Clear();
            _des = null;
        }
        //
        // Take this object off the finalization queue and prevent finalization
        // code for this object from executing a second time.
        //
        GC.SuppressFinalize(this);
    }

    /// <summary>
    /// Encrypts the argument string.
    /// </summary>
    /// <param name="inClearValue">String to be encrypted.</param>
    /// <returns>The encrypted string or empty string.</returns>
    public string Encrypt(string inClearValue)
    {
        byte[] buffer = ASCIIEncoding.ASCII.GetBytes(inClearValue);
        //
        string encrypted = Convert.ToBase64String(_des.CreateEncryptor().TransformFinalBlock(buffer, 0, buffer.Length));
        return encrypted;
    }

    /// <summary>
    /// Decrypts the argument string.
    /// </summary>
    /// <param name="encryptedValue">String to be decrypted.</param>
    /// <returns>The decrypted string or empty string.</returns>
    public string Decrypt(string encryptedValue)
    {
        byte[] buffer = Convert.FromBase64String(encryptedValue);
        //
        // Decrypt DES 3 encrypted byte buffer and return ASCII string.
        //
        string decrypted = ASCIIEncoding.ASCII.GetString(_des.CreateDecryptor().TransformFinalBlock(buffer, 0, buffer.Length));
        return decrypted;
    }
}

It will Help you.......

In the Decryption Class file you need to create one more method and in that you have to declare defualtly one value to that keyvalue. And pass that value as decryption method arguments.
like below
In Decryption Class page code.....

public Security()
  {
      lscryptoKey = "ChangeThis!";
  }

public String psDecrypt(String sQueryString,string key)
{
Byte[] buffer;
TripleDESCryptoServiceProvider loCryptoClass =new TripleDESCryptoServiceProvider();
MD5CryptoServiceProvider loCryptoProvider = new MD5CryptoServiceProvider();
try
{
buffer = Convert.FromBase64String(sQueryString);
loCryptoClass.Key = loCryptoProvider.ComputeHash(ASCIIEncoding.ASCII.GetBytes(key));
loCryptoClass.IV = lbtVector;
return Encoding.ASCII.GetString(loCryptoClass.CreateDecryptor().TransformFinalBlock(buffer, 0, buffer.Length));
}
catch(Exception ex)
{
throw ex;
}
finally
{
loCryptoClass.Clear();
loCryptoProvider.Clear();
loCryptoClass = null;
loCryptoProvider = null;
}
}
in Your .CS page code.....

protected void btnsave_Click(object sender, EventArgs e)
{
   if (txtusername1.Text.ToString() != "" || txtno.Text.ToString() != "")
   {
      objloginpl.username = txtusername1.Text.ToString();
      objloginpl.phoneno= txtno.Text.ToString();
      DataTable dtcheck = new DataTable();

      dtcheck = objloginbal.getforgotpsswrd(objloginpl);
      string str_decryptpwd;
      str_decryptpwd = objSecurity.psDecrypt(dtcheck.Rows[0]["password"].ToString(),"ChangeThis!" );

      //my doubt this line..
   }
   else
   {
      ScriptManager.RegisterClientScriptBlock(this.Page, this.GetType(), "Result", "alert('Please enter username or moblie no');", true);
   }
}