From the app home screen, just tap your programme title up top, or the "guides" icon on the top right corner, to see a list of the available options.
You can browse through the available programmes by swiping left/right. At the bottom of each programme page, you can select the frequency you would prefer (3 or 5 days per week).
Once you're done just tap the "select guide" button to switch programmes.
Please note that changing your programme will reset your completed workouts.