Just to let everyone know:
There is a maximum level of Mac OS that each version of Parallels supports installing. For example, with Parallels 10, it's Yosemite.
However, assuming that you have the installation install image**, although Parallels won't create a new VM if it's later than the maximum, you can upgrade an existing VM's OS FROM WITHIN THE VM ITSELF. In such a way, I've upgraded from Snow Leopard to El Capitan, then Sierra, each time after duplicating an existing VM so I could have all three. And all from Parallels 10.
(Do note that High Sierra breaks this, and you may need the current version of Parallels to create a HS VM.)
**it's good practice to download each new release from the App Store even if you don't install it; you can either store the image on your Mac or copy it to another disk, or even delete it as it will still show up under Purchases in the App Store and can be downloaded again anytime, even after it's superseded by another version of Mac OS.