Changing British Pounds (GBP) to US Dollars (USD) in Perch Shop PayPal

Is there a straightforward way to change British pounds to US dollars in the Perch Shop PayPal app? I tried just changing "GBP" to "USD" where it occurs in the two templates, product.html and product_in_list.html, but that didn't seem to have an effect. I'm thinking there is probably an easy way to do this but I didn't see it in the documentation.

Thanks for any help!