To customize titles / label values i.e. “Shipping Information” or “Zip Code” in Magento Onepage Checkout, open app/locale/en_US/Mage_Checkout.csv. This file lists the various labels used for onepage checkout in column A and their values in column B.
To change a value, simply locate the value that you want to change in column A and edit the text in column B to the text that you would like to be displayed for this label during checkout.
For instance, to edit the Shipping Information label, find “Shipping Information”,”Shipping Information” and replace with “Shipping Information”,”Your title”.
Remember to clear your cache in the Magento Admin Panel and then test the checkout to verify that the values for the titles / labels are displaying.