The content of your Apple Watch is automatically backed up to your paired iPhone. This allows you to restore your wearable device from a backup.
When you backup your iPhone to iCloud or iTunes, the data from your Apple Watch will also be included in that instance of the backup. Here is a full list of the items your Apple Watch backup will include:
– General system settings, such as your watch face, known Wi-Fi networks, brightness, sound, and haptic settings
– Time Zone
– Settings for Mail, Calendar, Stocks, and Weather
– App-specific data and settings, such as Maps, distance, and units
– Health and Fitness data, such as history, achievements, and user-entered data (To back up Health and Fitness data, you need to use iCloud or an encrypted iTunes backup.)
The backup doesn’t include all the data from your Apple Watch. For example, the Workout and Activity calibration data on your Apple Watch is not included in the backup. In addition, audio playlists synced to your watch, credit or debit cards used for Apple Pay, and your Apple Watch’s passcode will not be included in the backup.
If you ever have to unpair your Apple Watch from your iPhone, your phone will automatically create a backup of the data on your Apple Watch (the iPhone must be in range when unpairing for the backup to occur). When you are ready, you may restore the Apple Watch from a backup while pairing to a new device, however, for this to work you must be signed in with the same iCloud account or you must be using the same computer with iTunes that created the backup.
[via Apple Support]