Note: Please note that only Premium members can change between Meal Plans/Diets. Free users are offered the Standard Diet. When you start using the app, the Classic/Standard Diet is activated by default. 

Tap on the PLANS tab in the tab bar at the bottom of the screen and choose between the different Plans. If you have chosen a Diet and would like to change either the Diet or the Diet settings just go to the PLANS section again and choose the next Meal Plan/Diet.

This means that you cannot "delete" a Diet or Meal Plan - but if you ever leave the Classic Diet and choose another Diet or Meal Plan you can always go back to how you started off by choosing the Classic plan again.