You can upgrade the MacBook to another version through the App Store or using the Apple logo > Software Update. You can do it with the Parallels Desktop installed, but first you need to make sure that you have the virtual machines exported and available, because you will also have to update the Parallels Desktop as well. Luckily, the latest version of Parallels desktop is compatible with the latest version of OSX El Capitan.
Official website of Parallels Desktop: http://www.parallels.com/eu/products/desktop/
Apple Support page: https://support.apple.com/en-us/HT201541
