you can't just reverse a payment but you can dispute and try and get a refund. log onto your paypal account and then near the top of the page you'll see a tab for "resolution center." click on it and open a claim/dispute a transaction. there is a time limit to do this(i think 1 month from the purchase) to start this process so i wouldn't wait to long to get this going. there are a lot of help buttons and tutorials that will guide you along the process of starting a dispute so it should be pretty straightforward.
good luck bro and hope you get something outa this.