If you're using a trial version of Parallels Desktop, you can easily purchase the full version right from the application interface. To do so:
Click Buy in the About Parallels Desktop window or when you see the promo.
Choose one of the following:
Upgrade: Select this option to upgrade your current version of Parallels Desktop. Please keep in mind that you will need the activation key for your previous version. If the Upgrade option is selected by default, Parallels Desktop has already automatically detected the key for the previous version. If not, please make sure you have it.
Buy Full Version: Select this option to purchase Parallels Desktop if you have never used it before.
Follow the onscreen instructions. You may need to scroll down to see all the information.