"S" fares can't be upgraded. Only Y, B, H.

Try to locate your Iberia PNR (call AA if necessary) since it will be different from the one AA use and Manage your Booking on iberia.com. If you are eligible you should be able to find your way to the bid for the upgrade. As a codeshare perhaps you can't.

That reminds me of a potentially interesting thread - what, if any, benefits are there with booking a codeshare instead of direct with the operator. So many things get messed up due to the codesharing...