In Harry Potter 5 how do you learn the spell Reducto on PC?

You can't learn the spell "Reducto" in Harry Potter and the Order of the Phoenix on PC. That spell is introduced in the sixth book, *Harry Potter and the Half-Blood Prince*.

The PC version of *Order of the Phoenix* doesn't allow players to learn spells beyond those introduced in the book.

While there are mods and unofficial patches for the game, they are not widely available and not recommended for the average player.

However, you can learn Reducto in the later games in the series, starting with *Harry Potter and the Half-Blood Prince* and continuing in *Harry Potter and the Deathly Hallows - Part 1 & 2*.