Windows 10 has been out a couple of years now and hence Microsoft recommends that you don?t try to upgrade as the computer you?re trying to upgrade is likely to be 3 plus years old and in these days of technological obsolescence it?s time it was replaced. However there are plenty of computers 7+ years old that will run win 10 happily an so upgrades are possible. The official cost of Win 10 is at least ?50, depending on the version you buy and the media it?s supplied on. Depending on the computer there?s no guarantee that the hardware will still be compatible with Win 10, one of the other reasons MS recommend you buy a new machine. There are links to articles that say that you can still upgrade for free. This could be worth a go as it would save you the cash, which if the upgrade failed would have been a waste. Whatever you do make sure anything important is backed up before starting any upgrade. https://www.zdnet.com/article/heres-how-you-can-still-get-a-free-windows-10-upgrade/