How to Add a PayPal Button to replace Pay Now Button

Hi, I need to add a new PayPal button to my PreminResponsive Theme Directory website and replace the Pay Now button already on the website with the new PayPal Code Button. I did a properties search on the button that was there and this is what came up: javascript:document.checkout_paypal.submit

I can not locate the file and location to add the new PayPal Code to my website. Is there anyone that can explain to me how I can do this small change, I have the code already I need the location to add the code to the website.

Can anyone please help? Thanks in advance, Brian