You'll have to install iTunes. In an old thread from 2012, I've seen that, in order to get this done, you need Wine on your computer. Then you'll be able to install iTunes. There might be several videos in youtube, which will show you how to install Wine, and iTunes.I've only got ipad2 and old Xp computers now running ubuntu. All was a dream before upgrading to ios8. How could I upgrade vis itunes? It seems to me I can't. Any ideas most welcome
http://www.ipadforums.net/threads/how-to-use-itunes-with-ubuntu.69532/
Then connect your iPad to the computer, select your device in iTunes, click "Check for Updates", then "Download and Install".
As you're already up to date, restore your iPad. After installing the latest version of iTunes on your computer, backup your iPad (can also be done using iCloud), then restore it. This link will help with the procedure:
http://support.apple.com/kb/ht1414
Most, if not all of your problems might be removed this way. After updating, disconnect your iPad, then reset it: press and hold Home and Power button at the same time, until the Apple logo appears. Release both and let the iPad power up.
Hope that helps.