Help please my ipad has been jailbroken

connect it to the computer and update the os using itunes this should un jailbreak it

Try recovery mode. This will totally erase the device, which is what you need to do anyway.

You can read about it here or just follow the instructions below, copied from the support article.

http://support.apple.com/kb/ht4097

Recovery Mode Instructions

• Disconnect the USB cable from the iPad, but leave the other end of the cable connected to your computer's USB port.
• Turn off iPad: Press and hold the Sleep/Wake button for a few seconds until the red slider appears, then slide the slider. Wait for iPad to turn off.
• If you cannot turn off iPad using the slider, press and hold the Sleep/Wake and Home buttons at the same time. When the iPad turns off, release the Sleep/Wake and Home buttons.
• While pressing and holding the Home button, reconnect the USB cable to iPad. When you reconnect the USB cable, iPad should power on.
• Continue holding the Home button until you see the "Connect to iTunes" screen. When this screen appears you can release the Home button.
• If necessary, open iTunes. You should see the recovery mode alert that iTunes has detected an iPad in recovery mode.
• Use iTunes to restore iPad.

i have a jailbroken ipad air ios 7.1.2. i tried restoring it but my computer screen comes up with "unknown error occurred" need help

Did it give you a number with the error?

Put it in DFU mode (Recovery may not on occasion completely clear a jailbreak, use Google for instructions on how to do it) and then Restore it as a new device in iTunes.