If you want to claim VAT back you do this at the airport (you don't say where you are flying out of, but it really doesn't matter). You will need to have your receipt stamped at the VAT Refund office which is located landside. You may be asked to provide proof that the article is being taken out of the EU, so you'll need to show your ticket and be prepared to be able to produce the article(s) in question, so don't check your luggage in first.