We cannot change the skin of your pet, apologies. What skin your pet gets when hatching or fusing is random, so we cannot unlock another one for you.
If the skin you want it to be changed to is already owned by you, then you can do it by yourself via the pet wardrobe.
Additionally, we advise you to read our article on pet families to get a better idea of which skins you can get when hatching an egg or fusing a pet.