I’m playing Pokémon White, and I want to delete save data, I’ve already tried to press up+select+b at the same time but I would-be just see the normal screen. Does anybody know what I need to do? Maybe it is because I play it on my New 3DS?
You need to press Up+Select+B on the Title Screen, which is the screen where you see the title ("Pokémon White Version"), and the cover Pokémon (Zekrom).
The Title Screen should look like this (taken from Pokémon Black via an emulator):
Once you've pressed the buttons properly, the following screen should appear: