How to Sort the Contacts on Your iPhone
You can sort the contacts you have stored on your iPhone alphabetically — either by their last names or their first names. Choosing which method to sort your contacts allows you to find people quicker, based on whichever sorting method you prefer.
Tap Mail, Contacts, Calendars, scroll down to the Contacts section, and peek at Sort Order. Then tap Last, First or First, Last.
You can also determine whether you want to display a first name or last name first. Tap Display Order and then choose First, Last, or Last, First.