Therapy. There is a variety of approaches. They depend on the presence of pain and other symptoms. Those include range of motion exercises, ultrasound stimulation, heat/ice applications, massage, tens unit applications, stretching techniques. Physical therapist will be able to decide on which ones to use.
Consider ART. Active release therapy (art) can be very good for shoulder / neck combo. Be wary of anyone who wants to adjust or crack your neck. Stretching after a good cardio warm up is also key. And something you can do on your own.