As a chiropractor who specializes in posture and stretches to improve it (he sells his own 5 week posture program here: https://www.drkverneland.com/posture-programs/5-weeks-to-improved-posture), he has several stretches he recommends.
But if he has to recommend one it would Gravitational Chin tuck.
Lie on your stomach with your head over the edge of a bench or your bed. Gently tuck your chin in. Make sure your head is not extended backward as your head must be in line with the rest of your body.
Hold for 15 seconds and rest for 15 seconds.
How often: Repeat 5 times. Do twice every day (one time in the morning, one at night).
Why: It will help to strengthen the muscles that pull the head back into alignment over the shoulders (upper thoracic extensors) and also stretch the scalene and suboccipital muscles.
Dr. Kverneland founded Scandinavian Health Institute in Los Angeles in 2019. He has a doctorate from the Southern California University of Health and is in the process of being certified by the Institute of Functional Medicine. In addition, he is a sought after speaker in Europe and the US. His work has been featured in Martha Stewart, Thrive Global and Medium.