The words are in includes/languages/english/modules/payment/paypalwpp.php. There are lots of defines in this file. Yours may be different, but you could change MODULE_PAYMENT_PAYPALWPP_MARK_BUTTON_TXT (or whichever define matches what you're trying to change) to say something more like 'Using your PayPal account or a credit card'.

Note: I know that PayPal is particular on how their logo appears and what words are around it. I don't know whether this change is within their guidelines.

Another place I've changed some text is in includes/languages/english/TEMPLATE/login.php. The TEXT_NEW_CUSTOMER_INTRODUCTION_SPLIT and ...POST_INTRODUCTION... can also be used to guide new customers.