i need to update my iphone but it wont let me

You can't. You own an over 7 year old iPhone, whose hardware is not capable of updating past iOS 12.5.4. If you need the newer iOS versions, then it's time to retire that older phone and buy a newer, more modern iPhone.

You can't, sorry.

iPhone model 6 cannot support iOS versions beyond 12.5.4, because of hardware (CPU/RAM) incompatibility.

Regards

Giulio