if you want to redo the cehckout wizard, you would need to delete the pages created the first time you ran the checkout wizard:
Checkout.php
confirm.php
checkout _failure.php
checkout_success.php
shipping_failure.php
pp_confirm.php
pp_checkout_failure.php
pp_checkout_success.php
depending on your choices in the checkout wizard, some of these pages may not be created, delete any of thses pages that you find to start over.