How do I reset a restriction passcode I forgot on a iPhone 6 Plus

You backup your phone.

You restore it as a new device and set a new password.

https://support.apple.com/en-us/HT201304

“If you forgot your Screen Time passcode, update your device to the latest iOS or iPadOS then reset your passcode. If you can’t update your device, erase it and set it up as new to remove the passcode and choose a new one. Restoring your device from a backup won't remove your passcode.”