Updating Stystem Software

Hi,

I have a MacBook Pro (Retina, 13-inch, Early 2015) and i am trying to intall an app from the app store, it is telling me i cant intall the app because OS X version 10.14 or later is required. My MacBook says i have OS X El Caspitan version 10.11.6 installed but when i go into software update it tells me i have no updates available. How do i intall the lastest OS software so i can intall apps from the app store?