The "Complete Free Checkout" button is intended to reassure the customer that they will not be charged for that checkout in any way. Sorry, we have no way to customize the Free Checkout page, and because ours is a centrally-managed system shared in common among all our merchants, there is no way to customize our system for one merchant that would not also affect all other merchants just the same.
If you want to change that button because you are actually charging your customers, you should not be using Free Checkout (which only appears when your prices are set to 0.00). These are the payment processors we currently support, which are linked to our instructions for integrating them as checkout options in our cart:
http://www.e-junkie.com/ej/help.payment.processors.htm