Hi Tony,
To put PayPal Commerce in Sandbox mode you'll need to disconnect your account via its settings page. You then need to edit two files;
modules/plugins/paypal_commerce/admin/index.inc.php
Change:
define('SANDBOX', false);
to;
define('SANDBOX', true);
modules/plugins/paypal_commerce/paypal.class.php
Change:
private $_sandbox = false; // Set to false for live!!
to;
private $_sandbox = true; // Set to false for live!!
You can then reconnect using a developer Sandbox account from https://developer.paypal.com and test with their test or live ca