How can I reset my restriction passcode?

See this article. How to use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

Note the part which says: If you forget your Restrictions passcode, you'll need to erase your child's device, then set it up as a new device to remove the Restrictions passcode. Restoring your child's device using a backup won't remove the Restrictions passcode.

See here: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

You'll need to restore the iPhone. If you have a backup made prior to enabling restrictions you can use that, otherwise the restrictions passcode is stored in the backup, and will be put back when you restore it.