Best software upgrade for iMac 21.5" late 2009
Hello,
I have an older iMac and the software it came with was the snow leopard. As time went by I need to upgrade the software and upgraded to MacOS X Yosemite. that worked well until I did an update and my iMac stopped working. Geek squad helped me and did a back up to an external HDD and installed macOS Sierra since they blamed the OS X Yosemite update for corrupting and causing the issue. Apparently in order to complete my online classes i've recently enrolled in, I need to upgrade to macOS version of 10.13.0. The software I have now is macOS Sierra 10.12.6. I checked and there is no update. So what would be the best compatible software for my version iMac??
iMac, macOS 10.12