Where to change details on payment processor?
If you want the exact filename of the text, you can simply grep inside the civicrm directory - 

$ grep -rl "If you have a PayPal account" templates/
templates/CRM/Financial/Form/PaypalPro.tpl


Which gives the result of this file. You need to either - 


Change the Continue text and create a PR to get it accepted into core.
Override this file into an extension. (Not recommended based on Coleman's explanation on the link.)
Write a simple js in an ext and change the text of the paypal msg or the Contribute button.