1. Close the butterfly clasp. If the butterfly clasp is open, please fold it inward, one side at a time, until you feel and hear a click.
2. Press and hold the quick release button located inside the strap. All you have to do is hold down a button.
3. Gently open the chain link, and hold down the quick release button when pulling. Before taking off the strap of Apple Watch, you have to divide the strap into two parts.
4. Remove the strap, press and hold the strap removal button, then slide the strap horizontally to remove it, and then replace it.
Apple's official website
Do not press the strap into the groove. If you don't feel or hear the click, please slide the strap to the left and then to the right. If the strap is installed correctly, it will not slide at will unless you hold down the strap removal button. If the strap still cannot be locked, please put the strap in the middle position and push it into place. Then carefully twist the strap up and down. If the strap is sliding, don't wear an Apple Watch.