How can I change my passcode?

Your Apple (iCloud) ID password or the iPod's passcode?

To change your Apple (iCloud) password if you know the current password.

 Apple ID Password Change

 What to do after you change your Apple ID or password - Apple Support     I suggest doing this before you change your Apple ID or password.

If you forgot the password.

Enter the Apple (iCloud) ID and click Forgot Password.

Apple ID - Manage

Apple ID – Forgot Password

If the passcode for the iPod.

Passcode - Change and Help

Passcode - If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled