how can i chance my pet skin? because i can't select it it doesn't work but i want a another skin for my unicorn 

Please check FAQ for the basic gameplay questions like this. There's a lot of useful info already :)

1. How can I customize my pet?
   You can use your own art skills or skins created by other players to make your pet look unique. First you need to acquire a special magic item - Rainbow Jar. You can find it while exploring the islands and fulfilling the quests, and you can buy it at the Premium Shop. Then you need to use it in order to apply one of the public custom skins to your pet.

   • List of custom skins available for your active pet
   • Upload new custom skin

   Custom skin doesn't affect any genetics or pet characteristics. Children of a customized pet won't inherit the custom skin, they will be born with normal appearance.