You can use berries to lower EVs. They can be easily obtained in-game, and you likely have a few already. They are:
Pomeg - HP
Kelpsy - Attack
Qualot - Defense
Hondew - Sp. Attack
Grepa - Sp. Defense
Tamato - Speed
These berries lower EVs by ten each. Once the Pokemon has no EVs in the star and you use a berry, it will say something, then not use the berry. When every star is depleted, you can use super training in the desired stats or battle wild Pokemon that give out EVs for a certain stat. You can find many guides online for the latter.