I have an iMac from 2010 and want to update help

I purchased my iMac in 2010 and have the macos high sierra 10.13.6 and am wondering what I need to do to update this to 10.14??? My computer is showing there is nothing to update????? I just installed turbotax and it says I need to update my computer to 10.14 before install????? I have 500 gb storage and 4 gb memory! How can I update my computer???