How do I change my primary email address in 10.10.5 and iOS 8

I've accumulated several Apple IDs over the decades. The primary iCloud email is different on my mac from those on my iPad and iPhone. How do I change the primary email to make them all the same? IOW, I want to use the same Apple ID on my Mac, iPad and iPhone so they synch.

I'm using 10.10.5 on a 17" MBPro

TIA,

Bill