At the moment if there is a hardware issue with Face ID on the iPhone, Apple has to replace the whole device for a customer. This will change in the near future as Apple will offer Face ID repairs for the iPhone.
MacRumors has seen an internal memo that Apple has sent out to let authorized technicians know that they will have a new TrueDepth Camera service part that can be replaced.
This TrueDepth service part contains Apple’s Face ID and the front camera modules, so this can be used to fix and Face ID hardware issues that people are experiencing on the iPhone.
This will be available for a wide range of iPhone models including the iPhone XS, the iPhone 11, iPhone 12, and iPhone 13 models. The first handset to launch with Face ID was the iPhone X, but this model will not be available for repair, the repair module will only be available for the iPhone XS and above.
As soon as this feature is implemented you will be able to take your iPhone to the Apple Store and other authorized service centers to have the Face ID repaired on your handset. previously the whole device would have to be swapped for a new iPhone.