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#11
Install Windows 10 64-bit ISO from here. Use a utility to create a bootable USB Flash drive with that ISO, or burn to DVD if you can boot from a DVD. Backup all your data and then boot with that. Delete any partitions present and proceed to install Windows 10. No need to create any partitions, Windows Setup will automatically create the necessary partitions. When asked for a key click Skip. Then select Windows 10 Home or Windows 10 Pro, according to your present version (selecting the wrong version won't activate). After installation is complete install any drivers necessary and connect to the internet to activate.
Caution: Avoid using Diskpart as mentioned by the other poster. It is far too easy to make a mistake and lose data! Just backup your data, boot with the USB or DVD and follow the above steps.