The Start Over page lets you delete all of your data and reset everything to their defaults. It essentially wipes your login clean and acts as if you just signed up.
Settings at the top right side of the page, then click on Start Over in the red box at the bottom of the page.You can choose to delete everything and start over completely fresh using this form. All transactions, accounts, categories, budgets, reminders, etc will be completely removed. Accounts and categories will be reset to their defaults.
To do this, simply click the red Reset my account button. You will be prompted to confirm you want to do this and then the process will begin.