Hi samsmuthu,

After validation you need to check the password with 1234.

If it is 1234 then redirect to Change password page else redirect to the specified page.

Using this below article i have modified sample.

Simple User Login Form example in ASP.Net

ChangePassword.aspx

<asp:TextBox runat="server" ID="txtPassword" TextMode="Password" />
<asp:Button Text="Chage Password" runat="server" OnClick="OnPasswordChange" />

Login Page Code

C#

protected void ValidateUser(object sender, EventArgs e)
{
    int userId = 0;
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("Validate_User"))
        {
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@Username", Login1.UserName);
            cmd.Parameters.AddWithValue("@Password", Login1.Password);
            cmd.Connection = con;
            con.Open();
            userId = Convert.ToInt32(cmd.ExecuteScalar());
            con.Close();
        }
        switch (userId)
        {
            case -1:
                Login1.FailureText = "Username and/or password is incorrect.";
                break;
            case -2:
                Login1.FailureText = "Account has not been activated.";
                break;
            default:
                if (Login1.Password == "1234")
                {
                    Session["UserId"] = userId;
                    Response.Redirect("ChangePassword.aspx");
                }
                else
                {
                    FormsAuthentication.RedirectFromLoginPage(Login1.UserName, Login1.RememberMeSet);
                }
                break;
        }
    }
}

VB.Net

Protected Sub ValidateUser(sender As Object, e As EventArgs)
    Dim userId As Integer = 0
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As New SqlConnection(constr)
        Using cmd As New SqlCommand("Validate_User")
            cmd.CommandType = CommandType.StoredProcedure
            cmd.Parameters.AddWithValue("@Username", Login1.UserName)
            cmd.Parameters.AddWithValue("@Password", Login1.Password)
            cmd.Connection = con
            con.Open()
            userId = Convert.ToInt32(cmd.ExecuteScalar())
            con.Close()
        End Using
        Select Case userId
            Case -1
                Login1.FailureText = "Username and/or password is incorrect."
                Exit Select
            Case -2
                Login1.FailureText = "Account has not been activated."
                Exit Select
            Case Else
                If Login1.Password = "1234" Then
                    Session("UserId") = userId
                    Response.Redirect("ChangePassword.aspx")
                Else
                    FormsAuthentication.RedirectFromLoginPage(Login1.UserName, Login1.RememberMeSet)
                End If
                Exit Select
        End Select
    End Using
End Sub

ChangePassword Page Code

C#

protected void OnPasswordChange(object sender, EventArgs e)
{
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("UPDATE Users SET Password = @Password WHERE UserId = @Id"))
        {
            cmd.CommandType = CommandType.Text;
            cmd.Parameters.AddWithValue("@Password", txtPassword.Text.Trim());
            cmd.Parameters.AddWithValue("@Id", Session["UserId"]);
            cmd.Connection = con;
            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
        }
    }

    Response.Redirect("~/Login.aspx");
}

VB.Net

Protected Sub OnPasswordChange(ByVal sender As Object, ByVal e As EventArgs)
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(constr)
        Using cmd As SqlCommand = New SqlCommand("UPDATE Users SET Password = @Password WHERE UserId = @Id")
            cmd.CommandType = CommandType.Text
            cmd.Parameters.AddWithValue("@Password", txtPassword.Text.Trim())
            cmd.Parameters.AddWithValue("@Id", Session("UserId"))
            cmd.Connection = con
            con.Open()
            cmd.ExecuteNonQuery()
            con.Close()
        End Using
    End Using

    Response.Redirect("~/Login.aspx")
End Sub

Screenshot