Your SQL command is wrong. It is also a prime candidate for SQL injection. Please never, never do this on any query anywhere. This is always wrong.
First the SQL command issue. The syntax for an update is UPDATE <table> SET { <column> = <value> }+ WHERE <condition>
. In your query you don't set quantity to any value. This is going to generate a SQL error if you were to run the query itself. Furthermore you are appending the value of a dropdown to the product name clause and this is going to be wrong. You need 2 pieces of data here: new quantity and product name.
The SQL injection issue is because you never, ever concatenate values from a user into a SQL command. This makes it incredibly easy to completely destroy your database. Always use parameters, even when you trust the input.
The final issue is that you create a SQL command but never execute it. Hence the quantity will not change. I'm not sure what data access library you're using so I don't know the exact command you need to run.
using (var connection = new SqlConnection(CS))
{
connection.Open();
// Based upon your UI it appears you are displaying the quantity in a dropdown, so if the user selects 3 then they want 3 items so you
// need to subtract that from the current quantity (note you should probably ensure that multiple users don't subtract more quantity then you have)
var quantity = (int)dropdownlist1.SelectedValue; //Assuming it is an int here
var productName = "?";
var cmd = new SqlCommand("UPDATE ProductsDB SET Quantity = Quantity - @quantity WHERE ProductName = @ProductName", conn);
cmd.Parameters.AddWithValue("@quantity", quantity);
cmd.Parameters.AddWithValue("@productName", productName);
//Do the actual update
conn.ExecuteNonquery(cmd);
//Get the updated quantity from the DB using your regular query
....
}