How to update column in a gridview after saving data through text box?

 protected void BindGridView()
    {
        try
        {
            DataTable dt = new DataTable();
            dt = (DataTable)Session["userinfo"];
           
            gvPassInfo.DataSource = dt;
            gvPassInfo.DataBind();
        }
        catch (Exception ex)
        {
            //lblMessage.Text = DataObjects.Error_Message();
        } 
    }

    protected void gvPassInfo_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName == "change")
        {
            panelChangePassword.Visible = true;       
        }
    }

    protected void gvPassInfo_RowEditing(object sender, GridViewEditEventArgs e)
    {
        string user = gvPassInfo.DataKeys[e.NewEditIndex].Value.ToString();
        Session["user"] = user.ToString();
        panelChangePassword.Visible = true;
        txtUserid.Enabled = false;
        //txtUserid.Text = gvPassInfo.SelectedRow.Cells[0].Text;
        txtUserid.Text = user.ToString();
       
    }
 
    protected void btnSave_Click(object sender, EventArgs e)
    {
        clsUser objuser = new clsUser();
        //string user = Session["user"].ToString();
        string user = txtUserid.Text;
        string NewPassword = txtNewPassword.Text;
        string ConfirmPassword = txtConfirmNewPassword.Text;

        Response.Write("");
        BindGridView();
        panelChangePassword.Visible = false;  
    }