What will they do in physical therapy for my upper trapezius pain?

Different modalities: Massage, stretching, range of motion exercises, heat/ice application, vibration techniques, ultrasound, possibly traction, nerve stimulation if needed, iontophoresis - these are some but not all the modalities that can and might be used in the treatment of a trapezius pain. It will depend on the cause of the pain, training and comfort of pt therapist and your progress.

Decrease spasm: A physical therapy will be able to stretch out the muscles and break up spasm or "knots" in the muscle. They will also show you exercises that will help keep the muscles from tightening up on you.