updating a spreadsheet with new data from a separate spreadsheet

I need to update the values of the stock quantities in spreadsheet called "data old" with the stock values from the spreadsheet called "data new" .
The "data old" does not contain so many rows as the "data new". So i cannot just copy or paste also i also have 5000 item to keep updated .

The formula needs to look at the "data new" spread to see if a stock value has changed for a particular product code then replace that number in the"old data" spreadsheet .

The end result is i end up with a connection between the spreadsheets to keep my stock values up to date.




Thank you for any one who can help