This works the same way as the passcode that unlocks the device. You have to know the restrictions password in order to change it. Go to Settings>Restrictions>Enter the passcode. Then disable restrictions. You will have to enter the old passcode again in order to confirm your decision. After you have turned off restrictions, turn it on again and give it a new passcode.
IF you cannot remember the restrictions passcode, the only way to get rid of it is to restore the iPad as new. You cannot use the backup - unless the backup was created before the restrictions passcode was entered. Otherwise, you must restore as new which will totally erase the device. If you find yourself in this situation, and you have to restore as new, you can download most paid content again as long as you use the same Apple ID and as long as the content is still available to download again.
Download Past Purchases
http://support.apple.com/kb/ht2519