Unable to upgrade software beyond El Capital 10.11.6

Hi there!

I have a Macbook Air (13-inch, Mid 2013) running OS X El Capitan v 10.11.6

I need to update the os version as certain things (e.g. the updater for MS Office software) won't run any more unless I do so.

Unfortunately when I attempt a Software Update it says there are no newer software versions available for me to upgrade to.

I'd like to do the update manually but i'm not sure what version I can go to - can anyone advise at all?

Thank you! :)