The biggest advice with any iOS upgrade is to back up your data. You have two options: via iCloud or iTunes on your computer. I prefer the iTunes method because then my iCloud account space not taken up by the backup data. To back up your data through iTunes, here are the steps:
- Launch iTunes in your computer (Mac or Windows).
- Connect your iDevice.
- Select your device in iTunes and go to the Summary tab.
- Go to the Backups area and click on Back Up Now.
You may be asked to transfer apps from your device to iTunes. You can either do so or ignore it. You can always re-download any apps, but restoring them from to your iPhone, for example, will be faster if they are copied from the computer. I just re-download them. That way, you will always get the latest version.
iTunes will now have a copy of your data backed up on your computer. You can delete the backup at a later date if you see no issues with iOS 9.