Hi,
It's free up until July 29th (1st released), or there-abouts.
After that, you may/will have to purchase it. AFAIK.
&, if you are going to upgrade to WIN10, then it's highly recommended to back up your stuff beforehand.
The upgrade has been/is known, to cause a few "glitches/errors" & may even remove things it shouldn't.
Do you use Window's Media Centre by chance? (Not Media Player.)
If so, then the upgrade will delete it.
Anyway, check out the WIN 7 & WIN 10 fora for more info.
>
http://www.sevenforums.com/
>
http://www.tenforums.com/
Stay safe.