How do you get reducto in LEGO Harry Potter?

You can't actually get "Reducto" as a spell in the LEGO Harry Potter games. The games usually feature simplified versions of the spells, often with a different name.

For example, in LEGO Harry Potter: Years 1-4 and LEGO Harry Potter: Years 5-7, you might find a spell that's called "Exploding Charm" or "Bombarda" which can be used to break objects and sometimes even defeat enemies.

So, while you might not find "Reducto" directly, there are other spells that serve a similar purpose in the games. Just look for spells that cause explosions or break things!