Windows 10 from it's official website is free for download. Is using it to earn money online halal or it is haram in islam? How can I use it in halal ways without purchasing it?
You say this is from its official website. So it's not piracy, as Microsoft have given it to you freely. If you're concerned, read the terms and conditions to see if your usage is legal.
I'm not aware of anything saying you can't use something given to you freely, in the way it was intended to be used.