It's probably one of the first things you may want to do when you start customizing the theme to align its looks to your brand. To upload your logo, navigate to The Retailer > Theme Options > Styling.
What size should I use for the retina logo?
This second logo will be shown on retina display devices and that's why you should have in mind working with an image which has double the size of the regular logo. For example, if your main logo is 100x100px, the retina logo must have 200x200px.
This option can be found on the same page in Theme Options panel.