Bought second hand xbox 360 and it has a perental lock code on it how can I reset if it...

you will need to go to https://support.xbox.com/en-US/contact-us select your product, at the virtual agent select show other options (located below virtual agent)
select a topic (if necessary select a sub topic) then select show contact options and select callback.

The one that sells it will have to eliminate the blockade, since it must have what the user alex comments

• I assume this parental lock is for an account of the previous owner and as such you will have to Remove the xbox
  360 profile and add your own.

How to reset the pass code for parental controls on Xbox 360

If you have forgotten your pass code for your Xbox 360, you can reset it by answering the pass code question you created when you set up parental controls.

If you don’t remember your pass code question or don’t know the pass code for your Xbox 360, you can reset the pass code online. Here’s how.

Note You must meet the following criteria to be able to reset your pass code online:

• Your Microsoft account must be an adult account.
• The console must be registered in your name.

1. Sign in to your Microsoft account at
   Devices.
2. Locate your registered Xbox 360 console.
3. Select More actions, select Reset passcode, and then follow the instructions to reset your pass code.
   

We need additional infos, your account is a child one? the console is yours? you bought this console second hand locked with a code?

In other words... what's your scenario?

Because if you simply forget the code, you can try follow this instruction https://beta.support.xbox.com/help/xbox-360/security/forgotten-parental-controls-pass-code