Apple has discovered that a manufacturing run of the iPhone 6 Plus ended up using a faulty component in the iSight camera, causing photos to look blurry. (The iSight camera is the rear-facing camera.) Only a small percentage of devices sold between September 2014 and January 2015 are affected, but if your iPhone 6 Plus suffers from fuzzy photos, check its serial number at the iSight Camera Replacement Program for iPhone 6 Plus page to see if Apple will replace the camera for free. The easiest way to find the serial number is in Settings > General > About; tap and hold the number to copy it.
If your iPhone is covered by the replacement program, you can have it fixed at an Apple Retail Store or an Apple Authorized Service Provider; if neither is appropriate, contact Apple Technical Support. Don’t try to get help from your wireless carrier; they’ll refer you back to Apple.
Be sure to back up your iPhone to iTunes or iCloud before bringing or sending it in for repair. And if your iPhone has damage such as a cracked screen that would prevent the camera from being replaced, that will have to be resolved first, potentially incurring a service fee.
The replacement program covers affected iPhone 6 Plus devices for 3 years after the first retail sale of the unit, so even if your iPhone 6 Plus isn’t showing the problem now, if photos start to get blurry within that time frame, Apple will replace the camera.