This is an exclusive Premium Pass perk!
If you have activated yours, you are going to see a blue replacement icon at the end of every mission.
Simply tap the one you'd like to change and you are going to see a confirmation screen to make sure that you're replacing the correct one!
The first one of the day is always for free, so enjoy your perks, king of the Zoo! And, if you'd like to replace more than one, this will cost a few gems, okay?
And that's it! Enjoy this cool feature and see you at the Zoo! :)