Apple watch download stuck

Did you try to install the watchOS update directly onto the Apple Watch?

Have a look at this related article, which also has some more troubleshooting, for example deleting the update file in the Watch App on your iPhone.

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1. Make sure that your Apple Watch is connecting properly to its charger. 
2. Restart your Apple Watch.
3. Restart your paired iPhone.
4. Try again to update your Apple Watch.

If the update won't start:

1. Open the Apple Watch app on your iPhone.
2. Tap General > Storage.
3. Delete the update file.
4. Try again to update your Apple Watch.

Learn what to do if you see 'Cannot Install Update' when updating Apple Watch or if your Apple Watch says you need more available storage to install a watchOS update.

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copied from: Update your Apple Watch - Apple Support

First, make sure both your iPhone and Apple Watch are connected to Wi-Fi and have sufficient battery levels. Sometimes, a poor Wi-Fi connection or low battery can cause issues with downloading updates.

If the download is still stuck, try restarting your Apple Watch. To do this, press and hold the side button until the power off slider appears, then drag the slider to turn off your watch. After it's off, press and hold the side button again until you see the Apple logo.

If the update still won't download, you can try unpairing and re-pairing your Apple Watch with your iPhone. Before doing this, make sure to create a backup of your Apple Watch data as unpairing will erase all data on your watch. You can do this by going to the Watch app on your iPhone, selecting your watch, and tapping the "i" icon next to it. Then, choose "Unpair Apple Watch" and follow the prompts. After unpairing, you can re-pair your watch by following the on-screen instructions in the Watch app.

If none of these steps work, you may want to contact Official Apple Support for further assistance, as there may be a deeper issue with your device or network connection that needs to be addressed.