Windows 10

I bought a computer off Kijiji that was from someone who builds them for cheap. I bought it for a plex server.

I recently did a hard drive check, and the hd is getting spotty.

I did a check for windows license and it says it's "retail" with a partial key. I was thinking of buying a new SSD to put in but how do I get a windows key now?

I'm somewhat computer minded but this one I've never dealt with. (I also have no interest in installing Linux right now)

Thanks!