Note: Only Premium members can change between Meal Plans/Diets. Free users are offered the Lifesum Standard Diet, which is activated by default when you start using the app.
Tap on the DIETS 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 DIETS section again and choose the next Meal Plan/Diet.
How can I just remove the plan I am on? 
You cannot "delete" or remove a Diet or Meal Plan - if you want to go back to the standard version of the app you will have to go to the Diet tab and select Lifesum Standard