Change the iOS double tap home button reachabity predefined function
This isn't possible without jailbreaking. Apple does not provide any mechanism for changing the functionality of the double tap.

If the device is jailbroken, you can install Activator 1.9.11 beta 2 which has support for the ‘Double Press’ Touch ID event. This event can be mapped to ‘Home Button’, such that ‘touch fingerprint sensor twice’ will ‘simulate a press of the home button’.