1) To test using the sandbox you need to enter the Sandbox API Credentials.
2) When you are ready to go live, there are 4 server behaviors over 3 pages to edit.:
Authentication For PayPal Express Checkout
eCart Local Checkout
Get Payer Profile From PayPal Express Checkout
Process Transaction with PayPal Express Checkout
In those 4 server behaviors, you will need to turn of the sandbox and replace the sandbox api credentials with the live API credentials.