Visit the Bee Lab inside the game
Scroll to the right until you see Trait Swap
Tap Trait Swap
Choose the Bee you want to chanage
Tap "Cosmetic" (above the list of Bees) to choose the cosmetic you want to apply
Tap "Wear" under the Bee on the left
Congratulations on your new Bee!
CAUTION: If you swap out a trait, any boosts from the old trait will be lost.
NOTE: Chase Bees, one-of-one Bees, and some Mad Honey Vial Bees are not eligible for trait swap