PayPal Payments Pro Uses 4 server behaviors over 3 pages:
checkout.php:
Authentication For PayPal Express Checkout
confirm.php:
eCart Local Checkout
pp_confirm.php:
Get payer Profile From PayPal Express Checkout
Process Transaction with PayPal Express Checkout
In the server behaviors panel, double click each of these server behaviors to change the API Credentials used and toggle the Use sandbox option.