The software update can be downloaded through the dedicated Apple Watch app on an iPhone by going to General --> Software update.
To install the update, the Apple Watch needs to have 50 percent battery, it must be on the Apple Watch charger, and it must be in range of the iPhone. watchOS 3 also requires an iPhone running iOS 10 to install.
watchOS 3 introduces significant changes to the watchOS operating system. A new dock, accessible by pressing the side button, replaces Glances and houses 10 favorite first and third-party apps. Apps in the dock are kept continually up to date and are stored in memory for a new instant launch ability.
New Activity Sharing options allow Apple Watch owners to share workout and activity information with friends, and there are new fitness features for Wheelchair users. A Breathe app guides users through daily deep breathing sessions, an Apple Watch can unlock a Mac, and Apple has added apps for Reminders, Find My Friends, and accessing one's heartbeat.
Additional watch faces support complications, there are three new watch faces to choose from, including a new Activity watch face, and an SOS feature will make sure an option to get help in an emergency is always right at your fingertips. On the iPhone, there's a new Face Gallery for customizing Apple Watch faces, and watchOS 3 allows the Apple Watch to be located via Find My iPhone for the first time.
For complete details on the new features coming in watchOS 3, make sure to check out our watchOS 3 roundup.