How to - Updating iOS on iPhone 6 new from the box

I Have backed up via iCloud my wife's iphone 5 in ready ness to switch her over to her new iphone6+

Her old phone is running iOS 8.3, so when I try to restore. On the ne phone I get the message I cannot restore 8.3 onto 8.2,

whow what's the easiest way to upgrade the new iPhone 6+ to iOS 8.3? On start up iI have the options to set up the phone as another iPhone via iCloud or iTunes or as a new phone.

i don't recall seeing" there is a latest version of iOS for this phone" do you want to down load it.

where do I go to do this?

thanks