Haotmas posted this in response to an earlier question on how to do this:
"Step 1: From the Windows 10 Start Menu, click on Settings.
Step 2: On the Settings page, click on the button that reads Update & Security...
Step 3: Now on the Update & Security page, click on Recovery and then under Go Back to Windows X (where "X" is the version of your previous Windows prior to upgrading to Windows 10), click on Get Started.
Step 4: Select a reason for the rollback and then click Next.
After a warning and a confirmation, the rollback process will start. The process can take as short as a few minutes to about half an hour depending on the speed of your computer. In my experience, rolling back from Windows 10 to Windows 7 was much quicker (just about a fourth of the time) than my original upgrade process.
However, this downgrade option will be available only for one month since installing Windows 10.
After that, you may also want to consider installing a rollback tool like Easeus System GoBack. Because rollback option expires after one month. GoBack should work indefinitely."