Go to your account and look under 'Returns and Orders' and work from there. If bought from Amazon quite easy to sort returns, if from an Amazon Marketplace seller slightly more involved but easy enough.
emmie - it's quite simple. Go into Amazon and open "Your orders". Find the appropriate article and click on the box next to it "Return items" and take it from there. You are given many options as to how you want to return it, including having it collected from your home.