Set up and pair Apple Watch with iPhone
To use your Apple Watch Series 1 or later with watchOS 5, you need to pair it with an iPhone 5s or later with iOS 12 or later—Apple Watch models with cellular require an iPhone 6 or later. Setup assistants on your iPhone and Apple Watch work together to help you pair and set up your Apple Watch.
WARNING: To avoid injury, read Important safety information for Apple Watch before using your Apple Watch.
Turn on, pair, and set up your Apple Watch
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Put your Apple Watch on your wrist. Adjust the band so your Apple Watch fits closely but comfortably on your wrist.
For information about resizing or changing the band on your Apple Watch, see Remove, change, and fasten Apple Watch bands.
To turn on your Apple Watch, press and hold the side button until you see the Apple logo.
Bring your iPhone near your Apple Watch, wait for the Apple Watch pairing screen to appear on your iPhone, then tap Continue.
Or open the Apple Watch app on your iPhone, then tap Pair New Watch.
When prompted, position your iPhone so that your Apple Watch appears in the viewfinder in the Apple Watch app. This pairs the two devices.
Tap Set Up Apple Watch. Follow the instructions on your iPhone and Apple Watch to finish setup.
Tip: If you have difficulty seeing your Apple Watch or iPhone, VoiceOver or Zoom can help—even during setup. See Set up Apple Watch using VoiceOver or Use Zoom on Apple Watch.
While it’s pairing with your iPhone, your Apple Watch provides tips on how to interact with it. Tap Display, Digital Crown, and Side Button to learn more.
You can activate cellular service on your Apple Watch with cellular during setup. If you don’t wish to, you can activate it later in the Apple Watch app on your iPhone. See Use Apple Watch with a cellular network.
Your iPhone and Apple Watch must use the same cellular carrier. Cellular service not available in all regions.
Trouble pairing?
If you see a watch face when you’re trying to pair: Your Apple Watch is already paired to an iPhone. You need to first erase all Apple Watch content and reset settings.
If the camera doesn’t start the pairing process: Tap Pair Apple Watch Manually at the bottom of the iPhone screen, and follow the onscreen instructions.
Unpair Apple Watch
Open the Apple Watch app on your iPhone.
Tap My Watch, then tap your Apple Watch at the top of the screen.
Tap
next to the Apple Watch you want to unpair, then tap Unpair Apple Watch.
Pair more than one Apple Watch
You can pair another Apple Watch in the same way you paired your first one. Bring your iPhone near your Apple Watch, wait for the Apple Watch pairing screen to appear on your iPhone, then tap Pair. Or follow these steps:
Open the Apple Watch app on your iPhone.
Tap My Watch, then tap your Apple Watch at the top of the screen.
Tap Pair New Watch, then follow the onscreen instructions.
See the Apple Support article Use more than one Apple Watch with your iPhone.
Quickly switch to a different Apple Watch
Your iPhone detects the paired Apple Watch you’re wearing and automatically connects to it. Just put on a different Apple Watch and raise your wrist.
You can also choose an Apple Watch manually:
Open the Apple Watch app on your iPhone.
Tap My Watch, then tap your Apple Watch at the top of the screen.
Turn off Auto Switch.
To see if your Apple Watch is connected to your iPhone, touch and hold the bottom of the watch screen, swipe up to open Control Center, then look for the Connected status icon 
.
Pair Apple Watch to a new iPhone
If your Apple Watch is paired to your old iPhone and you now want to pair it with your new iPhone, follow these steps:
Use iCloud Backup to back up the iPhone currently paired to your Apple Watch (see iPhone User Guide for more information).
Set up your new iPhone. On the Apps & Data screen, choose to restore from an iCloud backup, then select the latest backup.
Continue iPhone setup and, when prompted, choose to use your Apple Watch with your new iPhone.
When iPhone setup completes, your Apple Watch prompts you to pair it to the new iPhone. Tap OK on your Apple Watch, then enter its passcode.
For more information, see the Apple Support article Switch your Apple Watch to a new iPhone.
Transfer an existing cellular plan to a new Apple Watch
You can transfer your existing cellular plan from your Apple Watch Series 3 (GPS + Cellular) to an Apple Watch Series 4 (GPS + Cellular) by following these steps:
While wearing your Apple Watch Series 3 (GPS + Cellular), open the Apple Watch app on your iPhone.
Tap My Watch, tap Cellular, then tap
next to your cellular plan.Tap Remove [name of carrier] Plan, then confirm your choice.
You may need to contact your carrier to remove this Apple Watch from your cellular plan.
Remove your old watch, put on your Apple Watch Series 4 (GPS + Cellular), tap My Watch, then tap Cellular.
Follow the instructions to activate your Apple Watch Series 4 (GPS + Cellular) for cellular.
For more information about setup and pairing, see the Apple Support article Set up your Apple Watch.
If you need to charge your Apple Watch before setup, see Charge Apple Watch.