Maybe you made a handful of changes to your settings and want to rebuild the whole weekly meal plan at once. Or maybe you just really hated the meal plan. You can rebuild the whole week by clicking "Regenerate week" at the top of the planner page on the website (you need to make sure you're in the week view instead of the day view to do this).
The planner takes only a few seconds to run, and will clear the existing meal plans on whatever date range you select. The only foods that will remain are ones that you've selected the "I ate this" checkbox on.