I believe the rules say we're not supposed to give instructions on how to use debug mode.
Either reset discs which you can buy with shards, or with ev reducing berries which can bought in the Department Store.
You can buy the Power Items in the Department Store, which give something like ten EVs in the chosen stat after every battle. Since most Pokemon can give up to 3 when you beat them this is a massive boost in quickly EV training Pokemon.