Bitrix Framework

The items, printed for user, in the form of all HTML generated by payment systems are segregated to an individual entity, similar to component templates. As the result, payment systems have some specific templates that you can change independently from payment systems. For example, if you do not like a standard invitation printed by a payment system, then you can re-define only this HTML and display your own invitation, without customizing the payment system handler itself.

You need to copy the template from payment system /bitrix/modules/sale/handlers/paysystem/<payment_system_name>/template/ to the site template /bitrix/templates/<site_template>/payment/<payment_system_name>/template/ and edit it as your require. The method \Bitrix\Sale\PaySystem\BaseServiceHandler::searchTemplate() is responsible for searching a HTML template.