C# - Updating The Quantity Through SQL?

I posted a question earlier about updating the quantity column in my table, I use the code found below however I think because the Select and Update is inside of a foreach loop it is updating all of the products.

Furthermore, the products are also updated when the page is reloaded and the amount is increased based on how many times the user clicks on the add button. E.g. Click the add button twice the quantity increments by two each time.

I ideally need to be able to use the ItemID outside of the foreach loop, but can't.

Code:

foreach (UserItem ItemID in (List<UserItem>)Session["UserSession"])
{
ConclusionPage.InsertCommand = "IF EXISTS (SELECT ItemID FROM tblUserItems WHERE UserID='@CurrentUser' AND ItemID='@ItemID') UPDATE tblUserItems SET Quantity = Quantity+1 WHERE (UserID = '@CurrentUser') AND (ItemID = '@ItemID')";
ConclusionPage.Insert();
}