Why do I upgrade apple watch 8.7.1, the app says it's preparing all the time?

Hello CuongNgo,

For difficulties with updating your Apple Watch you'll want to take the steps below:

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 > Software Update.

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.

You can also find those steps in the "Need more help with updating?" section near the end of this resource: Update your Apple Watch - Apple Support

Should that still persist, have you had the opportunity to try that process while connected to another Wi-Fi network? That should help to further isolate a potential networking conflict.

Let us know, and we'll keep moving forward from there.