I want to update price of an item in listbox

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C#

application

database

updater

, +

Hello Guys! i am working on entity framework database in my c# application but some thing here is so difficult like i click on image then product name and product price add in listbox but when i again add same item so item will add no issue with it price is not update in same product name
Here is the code:

Button b = (Button)sender;
                    tblProduct tp = (tblProduct)b.Tag;
                    string product = tp.productName;
                    lbProductsToBuy.Text = tp.ToString();
                    if (lbProductsToBuy.Items.Contains(tp))
                    {
                        MessageBox.Show("Items is already in listbox", "Duplicate", MessageBoxButtons.OK, MessageBoxIcon.Stop);
                        return;
                        lbProductsToBuy.Items.Add(tp.productPrice);
                    }
                    else
                    {

                    }
                    {
                        products.Add(tp);
                        total += (decimal)tp.productPrice;

What I have tried:

Button b = (Button)sender;
tblProduct tp = (tblProduct)b.Tag;
string product = tp.productName;
lbProductsToBuy.Text = tp.ToString();
if (lbProductsToBuy.Items.Contains(tp))
{
MessageBox.Show("Items is already in listbox", "Duplicate", MessageBoxButtons.OK, MessageBoxIcon.Stop);
return;
lbProductsToBuy.Items.Add(tp.productPrice);
}
else
{

}
{
products.Add(tp);
total += (decimal)tp.productPrice;