Apple is issuing a special version of iOS 9.2.1 today that is designed to unbrick iPhones affected by Error 53, according to TechCrunch. The company has also posted a support document that details the causes of and fixes for Error 53.

Error 53 crops up when you use iTunes to update an iPhone 6, 6 Plus, 6S, or 6S Plus with a TouchID button that has been repaired or replaced by a third party rather than an Apple-authorized repair center. Every iPhone is paired to the TouchID sensor that it ships with, and replacement TouchID sensors installed by third parties can only work as Home buttons and not as fingerprint scanners—this is primarily a security measure meant to detect and disable fraudulent fingerprint sensors. Phones stuck in a reboot loop because of Error 53 will be restored to otherwise normal operation by today's iOS update.

From Apple's statement on the issue:

“Some customers’ devices are showing ‘Connect to iTunes’ after attempting an iOS update or a restore from iTunes on a Mac or PC. This reports as an Error 53 in iTunes and appears when a device fails a security test. This test was designed to check whether Touch ID works properly before the device leaves the factory. Today, Apple released a software update that allows customers who have encountered this error message to successfully restore their device using iTunes on a Mac or PC. We apologize for any inconvenience, this was designed to be a factory test and was not intended to affect customers. Customers who paid for an out-of-warranty replacement of their device based on this issue should contact AppleCare about a reimbursement.”

The update is a new build of iOS 9.2.1, not a new version of iOS altogether, and it is only being issued through iTunes. Users with unaffected iPhones will not be receiving an over-the-air update, though presumably future versions of iOS (including version 9.3) won't introduce Error 53 if you get your Home button replaced at some point in the future.

If you've got a phone bricked by Error 53, plug it into your Mac or PC with a USB cable and apply the new update. If you need to put your phone into recovery mode manually for some reason, hold down your Home button as you power it on and wait until you see an iTunes logo before plugging it into your computer.