when sending back broken watch, how do I delete everything off of it first?

Turn off Activation Lock on a device

If you forgot to turn off Find My before you sold or gave away your iPhone, iPad, iPod touch, Mac, or Apple Watch, you can turn off Activation Lock using Find My iPhone on iCloud.com.

If you still have the device, see the Apple Support article Activation Lock for iPhone, iPad, and iPod touch, Activation Lock for Mac, or About Activation Lock on your Apple Watch.

1. In Find My iPhone on iCloud.com, click All Devices, then select the device you want to erase.
2. If you don’t see All Devices, it’s because you’ve already selected a device. Click the name of the current device in the center of the toolbar to access the Devices list, then select a new device.
3. Erase the device.
4. Because the device isn’t lost, don’t enter a phone number or message.
5. If the device is offline, the remote erase begins the next time it’s online. You receive an email when the device is erased.
6. When the device is erased, click Remove from Account.
7. 
8. All your content is erased, and someone else can now activate the device.