If you bought a outfit from a premium shop, you must go to npc Lora, The Witch. You can find her in all cities like antalis, maya, farson and schiva.
Lora, the Witch: Hello Player! I can change your outfit into an elf, dwarf, assassin, barbarian, elementalist, demonhunter, pirate, wizard, warrior, oriental, magician and shaman if you purchased them. What outfit do you want to wear?. You can also return to the basic outfits by typing "back".