Any way to revert back to an old OS on a Mac mini?

I was trying to load some photos a couple weeks ago and it asked me to upgrade the photos program before it would let me load any. Apparently I upgraded the whole OS in the process. The App Store screen popped up and I just hit update all. It is driving my wife crazy. The photos aren't ordered in any way and everything seems to run slower. Mac mini is about 2 years old.