Trying to Update to new OS

I don't use my Mac often, and it currently has OS X Yosemite 10.10.5. I am trying to update my phone on iTunes, and it says that I need OS X 10.11.4 or later.

I went to system prefernces, there is no software update link. I then selected About this Mac from the apple drop down, and selected software update from there, but nothing appears. I also went to the app store preferences as well to ensure that I am set up for automatic updates, which I am.

Any ideas how I can get the newest OS?