How do I backup ITunes on my IPad air?

You can't backup iTunes on your iPad. You can backup iTunes to your computer's backup disk using your backup utility. You can backup your iPad through iTunes either to your computer or to iCloud.

About backups in iCloud and iTunes

Create and delete iPhone, iPad, and iPod touch backups in iTunes - Apple Support

iCloud- iCloud storage and backup overview

Choosing an iOS Backup Method (Should I Use iTunes or iCloud to back up ...)

https://support.apple.com/kb/HT5262

Back up and Restore your iOS Device with iCloud or iTunes

http://support.apple.com/kb/ht1766

iTunes: About iOS Backups

http://support.apple.com/kb/ht4946

Create and Delete iPhone, iPad and iPod Touch Backups in iTunes

http://support.apple.com/kb/ht4946

iOS: Troubleshooting Encrypted Backups

http://support.apple.com/kb/TS5162