Return Apple Watch via apple care
Have received replacement & have boxed & bagged Watch to be returned but not sure where/how to post return back
iPhone 4s, iOS 9
watch Repair - Official Apple Support
I don't know about Watches but with other things Apple usually includes a label.
For instructions on how to 'Get your iPhone, iPad, or iPod touch ready for service' click here --> https://support.apple.com/HT201557
"There should have been a return label under the label used to ship the box to you. You should reuse the same box to return the device." https://discussions.apple.com/thread/252878948?answerId=255418639022#255418639022