You can switch the «Paypal Express Checkout» payment method to the sandbox (test) mode on the PayPal module settings backend section:
- Create a PayPal sandbox account adn get credentials for it.
- Set these credentials in the «API Username», «API Password», and «API Signature» fields in the PayPal module settings
- Switch the «Sandbox Mode» dropdown to the «Yes» state, save the settings, and clear the cache.