Problem in updating password field in C# form in SQL db via code in C#.net?

namespace Restaurant_Billing 
{
    public partial class changepassword : Form
    {
        public changepassword()
        {
            InitializeComponent();
        }
        SqlConnection cn = new SqlConnection();
        SqlCommand cm = new SqlCommand();
        
 
        private void button2_Click(object sender, EventArgs e)
        {
               Close();
                     
        }
 

        private void button1_Click(object sender, EventArgs e) //updating the new [
        {
            txtusername.Text = globalclass.m_globalvar;
            txtpasscurrent.Text = globalclass.m_globalvar;
 
            string username = txtusername.Text;
            string password = txtpasscurrent.Text;
            string newpassword = textpassnew.Text;
            string confirmnewpassword = textpassconfirm.Text;
            if (textpassnew.Text == textpassconfirm.Text)
            {
                cm = new SqlCommand("update tbluser set Pw = '" + textpassnew.Text + "' where U_name = '" + txtusername.Text + "'");
                cm.Connection = cn;
                cm.ExecuteNonQuery();
                MessageBox.Show("password has been changed");
            }
            else
            {
                MessageBox.Show("some errors!! dawg correct it!!");
                txtpasscurrent.Focus();
            }
        }
 
            private void changepassword_Load(object sender, EventArgs e)
        {
            this.txtusername.Text = globalclass.m_globalvar;
            cn = new SqlConnection(@"server=UMESHINATOR-PC\SQLEXPRESS;integrated security=true;database=nec");
            cn.Open();
        }
    }
}