This article only applies to the old designer. Not sure which designer you have? Check here.
Go to Design
From the merchant portal, choose Design from the navigation menu.
Go to the Designer
Find the section for Published Design and press the Designer button.
Go to the Checkout Page
At the top of the Designer, by the Cratejoy logo, is a Pages menu. Click this, and choose the Checkout page from the dropdown menu.
Edit the Checkout Page
From here you can make some modifications to the checkout page, including changing the image and some of the headers.
Save Your Changes
Press the Save button at the top right of your screen.
The markup and functionality of the checkout page cannot be edited.
Related: How to add fields for customer phone number/company name, Translating your checkout page to a different language