Modify iPhone restrictions passcode

Solution 1:

Erasing the phone in iTunes will remove the restrictions along with everything else.

Per my comment, you can change the passcode for restrictions under Settings > General > Restrictions

By clicking [enable restrictions] it will prompt you for password, you may disable it and then re-enter the passcode.

Reference as well here:
https://discussions.apple.com/thread/2478010?start=0&tstart=0

Solution 2:

From Techtips at salon.com:

1. Open the "Settings" menu from your Home screen, and then tap the "General" button.
2. Select "Restrictions" from the General page when it loads to the screen. A page loads prompting you for a passcode.
3. Enter the current PIN into the prompt. The iPhone will grant you access to the Restrictions preferences page.
4. Tap "Disable Restrictions" at the top of the References screen. The passcode prompt appears again.
5. Enter the current PIN. iPhone returns you to the Restrictions page.
6. Tap the "Enable Restrictions" button at the top of the page. Enter a new PIN into the passcode prompt when it appears on your screen. Confirm the PIN in the "Re-enter Your Restrictions Passcode" prompt to finalize the change.