My younger son has bought laptops in batches and is converting the good ones over to win 10 and flipping them....
He's found downloads for win 10 for free... He's been experimenting with the different keys and has found that you don't want to enter the key before you download and load the operating system as it will then only let you use the home edition... If you pass up entering the key before loading the operating system and enter the key after, you can get Windows 10 Pro loaded that has more features activated...