Recover iPhone SMS after updating iOS 8

Actually, there's a way to restore deleted SMS directly from iPhone 6/5S/5/4S/4/3GS.
If you've ever synced your iPhone with iTunes before, there is also another way: to recover from iTunes backup for all iPhones including iPhone 5S and iphone4S. You just need the help from an iStonsoft iPhone Data Recovery tool.

Part 1: Directly recover deleted photos from iPhone 6/5S/5/4S/4/3GS
1:Connect your iPhone to the computer
At first, please connect your iPhone to the computer and run the program after installing it.

2:Enter your iPhone's system and scan it (iPhone 5/4S users can skip this step)
After downloading the plug-in, you need to get into the device's scanning mode before scanning. You can follow the steps below:
1)Hold your iPhone and click the "Start" button.
2)Pressing the "Power" and "Home" buttons simultaneously for 10 seconds.
3)After 10 seconds, release the "Power" button, but keep holding "Home" for another 15 seconds.

3: Preview and recover iPhone messages
When the scan is completed, choose "Messages" to preview the detailed content. Mark those you want and click "Recover" to save them on your compute with one click.
Look more information:
How to regain deleted iphone sms
How to export sms from iphone backup