You can't change the asteroids in Veilstone into Deoxys in Pokémon games. Deoxys is a legendary Pokémon that can be obtained in different forms by using specific items and completing certain events.

It's possible you're thinking of the following:

* Deoxys in Pokémon Diamond, Pearl, and Platinum: In these games, Deoxys can be encountered at the Spear Pillar after defeating the Elite Four and becoming the champion. It appears as a rare encounter, not related to the Veilstone asteroids.

* Deoxys in other games: In other Pokémon games, Deoxys might have different ways of being obtained, like events, special trades, or using certain items.

If you're interested in catching Deoxys, it's best to check the specific game you're playing for the correct way to obtain it.