There is no flight mode option on the device but you can simply power off the watch when going on a flight.
Powering off the Device Remotely
To turn off the device remotely through the App, Tap Manage, Tap Turn off device remotely, Tap on the date box to set the time and date you want the device to automatically power back on and Tap the Powered off button.
To turn off the device so that it must be powered back on using the power button on the device, Tap Do not power on automatically which will turn green and Tap Powered off.
When the device is powering off you will hear the word “goodbye” from the speaker on the device.