I purchased a Plesk license, but the operating system I purchased it for is wrong. I would like to exchange the license for one that will work in my operating system.
It is not possible to change the supported OS of your Plesk license or exchange the license for another one. You should buy a new license.
Note that Plesk 12 licenses are OS-unified. Older Plesk versions are not OS-unified which means there are Plesk for Windows and Plesk for Linux license types.