Hi
I am sorry for lately response. I didn't get notice about your forum post. In case you haven't found the answer, here It is :
1. Login to back-end of your site .
2. Access to Payment Form -> Translation .
3. Find the language item PF_FORM_FOR (It sets to Payment for [FORM_TITLE] by default) . You just need to change the text to what you want (please note that [FORM_TITLE] will be replaced by the title of actual form) .
Thanks,
Tuan