Evaluate the effectiveness of an exercise program in reducing systolic blood pressure

A study is run to evaluate the effectiveness of an exercise program in reducing systolic blood pressure in patients with pre-hypertension (defined as a systolic blood pressure between 120-139 mmHg or diastolic blood pressure between 80-89 mmHg). A total of fifteen patients with pre-hypertension enroll in the study, and their systolic blood pressures are measured. Each patient then participates in an exercise training program where they learn proper techniques and execution of a series of exercises. Patients are instructed to do the exercise program three times per week for six weeks. After six weeks, systolic blood pressure are again measured. What type of nonparametric test should be used to evaluate the effectiveness of an exercise program in reducing systolic blood pressure?