From the Apple Support website:
iOS: Forgotten passcode or device disabled after entering wrong passcode
If you enter the wrong passcode six or more times, you'll see a message that says iPad is disabled.
If you have previously synced your device with iTunes, you may be able to reset the passcode by restoring the device:
Connect the device to the computer with which you normally sync and open iTunes.
Note: If iTunes asks you to enter the passcode, try another computer that you have synced with. Otherwise, go to "If you have never synced your device with iTunes, or you do not have access to a computer" section below.
If you have never synced your device with iTunes, or you do not have access to a computer
- If the device is still disabled, or if iTunes does not automatically sync your device, sync the device with iTunes.
- When the backup and sync is complete, restore your device.
- When iOS Setup Assistant asks to set up your device, choose "Restore from iTunes backup".
- Select your device in iTunes and choose the most recent backup of your device.
If you see one of following alerts, you need to erase the device:
iOS Forgotten passcode or device disabled after entering wrong passcode
- "iTunes could not connect to the [device] because it is locked with a passcode. You must enter your passcode on the [device] before it can be used with iTunes."
- "You haven't chosen to have [device] trust this computer"