I know of no way to "downgrade" Windows, it simply won't let you. You will have to format and reinstall Windows 7.
You can however keep your files by backing them up to an external or internal storage hard drive that would not be formatted by Windows. The same can be done for your saved games files, you will have to find in their respective folders, which might be in programs files, my documents or in your user files. You can google specific games information to find out where they are. Steam games have the option of being backed up through steam itself. It will save you the download time by backing them up to your storage hard drive.
After reinstalling Windows and your games, you should be able to transplant your saved game files to their folders and continue gaming.
Another word of advice, Windows 7 now like 10 has "spyware" through Windows updates; you can search to see which current list of updates track location and other user information and do not install those updates.
It is a time consuming process to change everything back over, plan time to do the main work then your computer should settle back.