The Magento checkout “What is PayPal” text is code in this file, as well as paypal logo
Ideal way to make changes is copy this file with same path in your theme and modify this file according to requirements.
Example your theme is: yourtheme
Then in template folder create same path as “paypal/payment” and put modified mark.phtml here
If the path and file is already here you can directly modify it.
After modify, you should see, example: