Piece on back has come off. How do I make it stick?

Piece on back has come off. How do I make it stick? In the center of the back of the watch is a small circular cover for the sensor. I did not realize it was a separate piece until it came off on my wrist when I took off the watch. I am wearing it without the piece and the power drains and not all functions work. I would like to repair this. Thanks.