iOS 10.1 is a free download, available either through iTunes or over-the-air via the Settings > General > Software Update menu in iOS.
The new Portrait mode takes advantage of the twin cameras on the Plus, using them to gauge the depth of a scene and blur the background. Apple announced the mode when it revealed the phone on Sept. 7, but said it would only be available in a later software update.
Apart from the new photo option, changes should include a fix for CallKit issues that were preventing call-directory extensions from working. Caller ID and blocking extensions were unreliable. In Japan, the software adds Maps transit directions for "every major train, subway, ferry, and national bus line," but local bus coverage is limited to Tokyo, Osaka, and Nagoya.
Apple has simultaneously released tvOS 10.0.1 for the fourth-generation Apple TV, and watchOS 3.1 for Apple Watches. The updates mostly offer bugfixes and performance improvements, though watchOS 3.1 users can now replay bubble and fullscreen effects in Messages, or see Messages effects even with the Reduce Motion feature enabled —this change mirrors iOS 10.1.