How do I upgrade from Mac OS X Yosemite 10.10.5 to OS X El Capitan 10.11.4?

I would like to accept the iOS 12 upgrade to my iPhone that the iTunes app on my MacBook Pro keeps reminding me about. But when I click on the Download and Update button, I receive an error message telling me that I need to upgrade my system (OS X Yosemite 10.10.5) to OS X El Capitan 10.11.4. But when I try to download El Capitan from the App Store, I get the following error message: We could not complete your purchase. This version of OS X 10.11 cannot be installed on this computer.

Any ideas on how I can upgrade from OS X 10.10.5 to 10.11.4 on my MacBook Pro?

Wayne