I don't know of a downgrade as such, other than maybe rolling back from Win10 to Win7 if you upgraded very recently (like a month).
If you bought a computer in the last couple years with Win10 pre-installed, it might be a challenge to install Win7 on it, because of specialized drivers and so forth. Win7 was released in 2009 with SP1 in 2011. It already went out of normal (feature) support, and Microsoft pressured the manufacturers to stop providing Win7 support after Win10 came out.
So a lot of newer Win10 computers won't run Win7 with full functionality UNLESS you are willing to go hunting around for third-party drivers. In most cases it can be done, but it's a bit of work.
“If you bought a computer in the last couple years with Win10 pre-installed, it might be a challenge to install Win7.”
You’re right — and I learned that too late. For Christmas I’m checking HP and Dell, both of which sell Windows 7 systems in their Business section.