Bottom right, click pets, scroll to the pet you dont want, click the free button.
Depending on the number of stars you get some rewards back.
1-2 stars: NUTHIN
3 Stars: 1/3 x Large EXP Pet Pots
4 Stars: Kindling/Spiritual Fruit/Spirit Crystal/???
5 Stars: 1 x Mystery Summon (Regardless of VIP)
Wait.. why are you releasing 5* pets? Sometimes you get extra pets. I have 3 archangels -.-