In-game head over to the shop and click on Skins. You will need the Crystal currency to unlock some skins. The easiest way to get the paid currency is refer a friend or be referred. You can refer my code to get 1000 Crystals which gives just enough for a skin. Other ways to get Crystals is to do quests and or purchase them.
Copy and paste or type in the number below into the Redeem Bonus Code and hit enter for the rewards.
5257360
Once you get your rewards buy one of the cheaper skins for 950 crystals and the achievement will pop.