How Do I Change Default Text

Hey Guys,
I'm setting up MarketPress for a service-oriented business and I need to change some of the default text.

First, since this is a service-oriented business, I'd like to replace the word "product" with the word "service" (and "products" with "services") on whatever PAGES make reference to it. Similarly, I'd like to replace the word "shipping" with "customer". So that, for example, headings that currently read "Shipping Information" read "Customer Information" in the future.

The changes need to be permanent so they aren't overwritten by future updates.

Thanks