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Open Parallels and boot Windows 7. While Windows is open, click on Actions menu at the top bar and do you see "Upgrade to Windows 10" option. See this picture --> http://kb.parallels.com/Attachments/kcs-36831/Screen%20Shot%202015-08-20%20at%2004.58.21.png
If not, download the Windows upgrade tool on Parallels, open it and follow the onscreen instructions to upgrade.
On Windows, go to this website --> https://www.microsoft.com/en-in/software-download/windows10
If the above tools doesn't work, then download the following tools below (on Windows)
Download Tool Now (32-bit version)
Download Tool Now (64-bit version)
Click Upgrade Now and a tool will be downloaded. Open it and follow the onscreen instructions to upgrade. Remember, you need to have atleast 2GB RAM assigned to parallels for Windows 10 to be installed.
If you have any difficulty on following the steps above, please let me know.