Arranging icons on iPhone 7 plus

You can arrange them manually the same as on earlier versions. With a phone that has 3D Touch you need to touch and hold an app lightly in order to start all of the apps jiggling. If you press too hard it may bring up a menu rather than jiggling the apps. Once they are jiggling you can drag them as desired. Press the home button when finished to stop the jiggling.

If you want them in alphabetical order, you can go to Settings > General > Reset > Reset Home Screen Layout.