That is a completely different can of worms. There are raging debates about the availability and legality of copies of Windows 7 -- suffice it to say that Microsoft doesn’t have any legal source of the bits for individuals. If you’re very lucky and you have the right kind of key, you can download an ISO of Windows 8.1 on an official Microsoft site.
I had a friend stuck in a similar situation, where Windows 10 was unstable. Rolling back from Win10 to Win7 left him with a system that constantly crashed. My suggestion: Back up his data as best he could, rerun the upgrade, then go to Windows 10. Inside Windows 10, run a Reset (Start > Settings > Update & security > Recovery), then “Remove everything / Removes all of your personal files, apps and settings.” That triggers a clean install of Windows 10. He may not like Windows 10, but running that clean install made it substantially more stable. He learned to live with it.
Your mileage may vary, of course.
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