In the battle frontier, there is this guy who says:
I've collected 197 BP. Just 3 more, and I can change my Druddigon's Ability!
Is it really true that you can change Pokemon's abilities with BPs? If possible, how & where can I change it?
Yes you can.
If you can get 200bp you can buy an ability capsule, which switches a Pokemons ability to its other one(only works with Pokemon that can have 2 abilities)