If you don't have Windows 7/8 installed which would allow you to upgrade to 10 free of charge, then you can simply install Microsoft's Windows 10 on your PC as it is (via a USB or DVD).
Are there any limitations? You just won't be able to customize, and that's it. I'd say for most people that's not even an issue, anyway.
It will look something similar to this below with that watermark on the bottom right.
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Chief Bargain Hunter