go to the security centre links on the main page (once you logged in)
but paypal will investigate and if they get nowhere with the seller you will get you money back, but it takes time
also Kaedesblade highlighted recently that the seller gave a previous tracking number for a different item he was trying to chargeback and because that number checked out - paypal ceased the investigation
the other option you have is to contact your credit card company, they will refund you and seek the money from paypal - paypal will bollock you for it - but hey you have your money right
if you paid with a debit card - no chance, my debit card company told me because i paid paypal and no the seller directly i wasnt entitled to a refund because paypal honoured the agreement by sending payment to the thief rat bastard who stole from me