If you have the pokemon in the first slot of your party and talk with it, it shows how much friendship your pokemon has.
Less than 50 is “hates travelling”
50-100 is “undecided”
100-150 is “is happy traveling”
150-200 is “loves travelling”
200+ is “loves nothing more than travelling”
Most pokemon evolve at 220+ happiness/friendship.
Like Fel said, you can use the EV reducing berries from the department store, it’s the quickest way and they are quite cheap. Alternatively, you can use Vitamins or have it hold a Soothe Bell and battle or run around a lot with it while having it at the front of your party(If/when you battle, make sure that it doesn’t faint).
To decrease happiness(for delta buneary min happiness evolution), fainting it repeatedly will decrease happiness. Using bitter medicines will decrease happiness as well, they can be found in the Black Market(excluding revival herb) as well as Narra town.