While Quicken Simplifi does not have a feature that will delete all of your data, categories and tags included, and start over, you can delete all of your accounts manually by:
Hover over the left side panel and click Settings.
Click Accounts.
Locate the bank that you want to delete and click the three dots at the end of it.
Click Delete institution, then Delete to confirm.
Click here to learn how you can delete a category or a subcategory or click here if you need some help deleting a tag!