How do I downld free mincraft for wndows 10?

It's impossible. You will get viruses on your computer. If you can't purchase it, try some browser-based .io games like slither.io, agar.io, evowars.io, and so on.

There's no legal way to download Minecraft for free. Try out Minetest. It's free, and has a large number of servers and mods available.