Update ListItem into SharePoint List using Server side code.

SPWeb currentWeb = SPContext.Current.Web;

SPList lstLogin = currentWeb.Lists["Login"];

SPListItemCollection myColl = lstLogin.Items;

foreach (SPListItem item in myColl)

{

       if (item["UserName"].ToString() == txtUserName.Text)

       {

              item["PassWord"] = txtPassWord.Text;

              item.Update();

              lblMessage.Text = "Updation Successful";

              Clear();

              break;

       }

}