This question already has an answer here:
I have an iMac12,2 with Mac OS X 10.6.8. I want to upgrade to the newest possible Mac OS. If I click the Software update button it says "Software Update doesn't have any new software for your computer at this time.". If I want to download High Sierra from the App Store it says "This update requires macOS version 10.8 or later."
What should I do? How can I upgrade my mac?