You may try this option to upgrade to iOS 9. That is the easiest option but your device should have enough space. If you do not have enough space, here is how to install iOS 9.
- To manage space on your device, tap Settings > General > Usage > Manage Storage.
To do this, you need to plug your iPhone or iPad into your computer and update using iTunes. Here is how:
Note that you may want to backup your device before installing iOS 9 on your device.
1- Install iTunes, if you have not already.
2- Plug your device into your computer, PC or Mac, using your USB cable.
3- Launch iTunes. You may see this message: “iTunes could not connect to the iPad “xxxxx’’s iPad” because it is locked with a passcode. You must enter your passcode on the iPad before it can be used with iTunes.” If you do, on your device, enter your passcode and tap “Trust this Computer”; then click Try again in iTunes
4- In iTunes, select your device, then click Check for Update.
5- Click Update. You will see a message saying “iTunes will update your iPad to iOS 9.0 and will verify the update with Apple.” click Update.
6- Click Next then Agree to Terms.
7- You may need to enter your passcode. This will start the download. This may take to several minutes. After it is done, iTunes will say “updating iPad (iPhone) software…” This means that update process was started. This may also take several minutes.
8- When it is done, complete the process on your iPad. You will see the “slide to set up” screen. Just follow the steps. Done. Enjoy iOS 9.