Install macOS High Sierra with a patcher tool
It’s possible – but not particularly easy – to install macOS High Sierra on a Mac that’s too old to support it officially. A hacker known as DOSDude1 has written a patch tool that can install
High Sierra on Macs manufactured as long ago as 2008. There’s a tutorial and a download link on his website at dosdude1.com/highsierra.
Think carefully before you use it. There’s a reason why High Sierra isn’t supported on those Macs. It’s likely to run less than optimally, especially on lower-powered Macs, so you might be better off leaving your Mac on an even older version of macOS. It’s fun to do if you’re up for it, though.