It's not currently possible to 'reset' an account once data has been added. You will need to manually go through and delete each entry. Because this can be a slightly tedious process depending on the amount of test data, we generally suggest deleting the test account and creating a fresh, new account.
You can delete your account at any time under 'Settings > Account > Delete account'.
Unfortunately, we cannot do this on your behalf. Deleting the account is permanent and will delete all data, which cannot be recovered. Once the account is deleted, you can sign up for a new account with the same email address.