Changing text on checkout fields in marketpress

Hi

I have a multisite install of marketpress.

on one of the stores the client would like to take manual payments in the form of a deposit.

we are doing this by sending a link to a paypal payment page in the email from marketpress.

I would like to be able to collect the contact details for the customer in the shipping info page. To do this I would like to change the title of “Contact Information” rather than shipping details.

Can you tell me where I could do this on a per site basis, so that other sites are not changed in the multi site install.

thanks

Ian