The box came to return the old watch but no seal or label

To return your item(s) to Apple, sign in to online Order Status with your Apple ID and password. 

After you sign in, click the Edit Orders button to view your most recent order. Locate the item you want to return and click the Return Items link. On the next page, select the item(s) you would like to return and click the Continue button. 

Next, verify the return item(s) and the return address on your address label. Once you have verified your return details, click the Print Now button to print your address label and packing slip. Affix the address label to the outside of your return box. If you have multiple items to return, you may pack them in a single box with the packing slip.

Returns & Refunds - Shopping Help - Apple

You might need to print the label