After a little bit of scanning and searching the plugin files… I figured it out.
I think this is correct. It works for me.
This will change it on all WooCommerce pages too..
Use a child theme functions.php
add_filter( 'woocommerce_product_single_add_to_cart_text', 'ql_replace_add_to_cart_button_text' );
add_filter( 'woocommerce_product_add_to_cart_text', 'ql_replace_add_to_cart_button_text' );
add_filter( 'mkl_pc/add_to_cart_button/default_label', 'ql_replace_add_to_cart_button_text' );
//Replace Add to Cart text with Buy Now!
function ql_replace_add_to_cart_button_text() {
return __( 'Purchase', 'woocommerce' );
}