To back-up or restore you contacts in the addressbook you can do for both the following steps :
- Open the TronWallet app
- Tap on the most right icon on your navigation tab bar to go to SETTINGS
- Tap on ADDRESSBOOK
- Only for TW3 users : tap extra on TRX button
- Tap on the SYNC BUTTON on the top right
Now your contacts are synchronized with the blockchain smart contract of your wallet. You only have to hit that button 1 time.