Before replacing the battery should a backup be performed? What is th

It is important to do a backup before any repair on a device. You have two options, both work just as well:

1. Using iTunes, it does not back up (but doesn't require an internet connection to back up or restore):

Content from the iTunes and App Stores, or PDFs downloaded directly to iBooks (click transfer purchases to do so).

Content synced from iTunes, like imported MP3s or CDs, videos, books, and photos.

Photos already stored in the cloud, like My Photo Stream, and iCloud Photo Library.

Touch ID settings

Apple Pay information and settings

Activity, Health, and Keychain data (use an encrypted backup with a password to do this).

2. Using iCloud, it does not back up (but requires an internet connection to back up and restore):

Data that's already stored in iCloud, like Contacts, Calendars, Notes, My Photo Stream, and iCloud Photo Library.

Data stored in other cloud services, like Gmail and Exchange mail.

Apple Pay information and settings

Touch ID settings

iCloud Music Library and App Store content (you can just tap to download all your apps again).

- This unit has Storage which is non-volatile memory.