Install macOS Mojave on Older Macs
Apple have released macOS Mojave, and if you would like to install it on a unsupported system, check the tool below.
Obviously no warranty or support are given for this tool.
Install macOS Sierra on Older Macs
If you are sitting there with your old Mac feeling left out in the cold there is a way to shoehorn macOS Sierra onto your ageing computer.
Check out the macOS Sierra Patcher Tool for Unsupported Macs. And follow the instructions. This is of course not endorsed or supported by Apple in any way.