Mega Greninja is not officially available in the mainline Pokémon games
Greninja cannot Mega Evolve in official games
Use Ash-Greninja in Pokémon Sun, Moon, Ultra Sun, Ultra Moon, or Pokémon Battle Bond mechanics in supported games
If you mean a fan game or mod, check that game’s specific evolution or transformation method
If you mean Pokémon GO, Mega Greninja is not currently available there
If you mean Pokémon HOME or trading, no official trade can obtain Mega Greninja because it does not exist officially