How do I cancel the extra storage I paid monthly on my old iPhone 6. I just upgraded to iPhone 14

Firstly, to be sure you're asking the right question, the storage you purchased was a subscription to Apple's iCloud+ and not to increase the capacity of your device.

iCloud storage is access to 50GB, 200GB or 2TB of space on Apple's servers for you to house data for your iCloud account. You cannot increase the capacity of your device. Apple explain the difference in What's the difference between device storage and iCloud storage?

If you wish to cancel your iCloud+ storage plan you can cancel it by downgrading to the free 5GB plan. See how to do this in Downgrade or cancel your iCloud+ plan.