Apple watch series3不能更新os7.4

1. 取消 Apple Watch 配对，此时 Apple Watch 会自动备份。
2. 重新配对 Apple Watch 并选择设置为新 Apple Watch，而不要恢复备份，且不要安装任何 App。
3. 更新 watchOS。
4. 如果需要恢复备份（其实也没啥需要恢复的），请再次将 Apple Watch 取消配对，然后在重新配对时选择从步骤一的备份恢复。

参考：https://developer.apple.com/documentation/watchos-release-notes/watchos-7-release-notes

• You might be unable to update Apple Watch Series 3 to watchOS 7 due to limited storage. (64793483)
• Workaround: Unpair your watch, then pair it again with your phone. Set it up as a new watch rather than restoring from a backup. Use General > Software Update in the Watch app to update to watchOS 7. If you want to restore your watch from its latest backup after the update is complete, repeat these steps while choosing to restore from a backup rather than setting up as new.