Restrictions code

These are the minimum requirements to reset the ScreenTime passcode:

• The device must run iOS/iPadOS 13.4 or later;

AND

• You must know the Apple ID & Password that were used to set up that ScreenTime Passcode.

If you meet both above requirements, these are the instructions:--> If you forgot your Screen Time passcode – Apple Support

But since your iPhone 6 cannot be updated to iOS 13.4 or later, you need to erase that device following these instructions (even if related to the passcode to lock/unlock device's screen):--> If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

The bad news is that you cannot restore data from a backup, since the ScreenTime passcode would come back in.

Regards

Giulio