With Restore option using iTune, What happen to the existing content?

The content will be totally erased. One will typically choose the restore option if the device as a last resort because of problems that can not be resolved any other way. Once the device is erased a fresh copy of the iOS will be downloaded and placed on the device. This is called restoring to factory settings.

The good news is that restoring to factory settings is a standard troubleshooting step that can clear up all of the problems on your device and if you have a backup, you can restore the backup, sync with iTunes again or load your iCloud backup and the device will be exactly like it was before you restored the iOS.

Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support

When I first tried to update my iPad 3 to iOS 9, I got the connect to iTunes screen. I chose the option to update again and it went fine, I didn't have to restore. I would give that option a shot first and if you need to, then restore to factory settings,

It's too late to backup now. However, when you asked what happens to your content, I assumed that you meant your data on the iPad. You can not save the data or settings now. But you will be able to download all of your purchased content again for free as long as it is still available to download. This explains how to do it.

Download past purchases from iTunes - Apple Support

I'm sorry, but no. It is too late now to recover your files. If you were using the iCloud Photo Library, your photos will still be in iCloud and should download to the iPad after you restore it. If you were not using iCloud Photo Library, and don't have the photos backed up anywhere else, they are gone now.

Sorry, I had to leave the discussion last night. I don't know where you are in this process, but like I said, I would choose the update option and see if it works. If that will not work, then you will have to restore the device.