How do I force a hotspot IP address change on an iPhone6?

The IP addresses handed out by an iPhone through its Hotspot to other devices connected to it cannot be changed, but as they are just for use with the hotspot, they should not require changing.

Why do you need to change them?

The IP address handed to a device from an iPhone Hotspot, is not sent to Facebook. Facebook only gets the pubic address of the iPhone providing the hotspot which is assigned by the cell carrier the iPhone is using.

To Facebook all devices connected to an iPhone Hotspot will have the same IP address.

Just like all devices connected to a Wifi router will only ever appear to Facebook from the Router's public IP address.

Facebook can only ever see the public IP address. The internal network address is never revealed to it.