Incorrect syntax near 'System'.

protected void Button1_Click(object sender, EventArgs e)
        {
            SqlConnection con = new SqlConnection("data source=.;database=MRC_Database;integrated security=true;");
            con.Open();
            SqlDataAdapter da = new SqlDataAdapter();
            string updatestr = " Update User_Account set F_name=" + txtFname + ",L_name='" + txtLname + ", phone_no ='" + txtPhone +
                ",PassUser='" + txtNewPass + ",ReEnter_password='" + txtReEnterPass + ", AlternateEmail ='" + txtAlternateEmail +  
                "' where Email='" + LblEmail + "'";

            SqlCommand com = new SqlCommand(updatestr, con);
            da.UpdateCommand = com;
            com.ExecuteNonQuery();
            con.Close();

        }