When you log in to your account, where you download the high resolution versions of your logo, there is a link to "edit logo." If you click that, it will allow you to edit your logo. When you save your edits, your logo is automatically updated on the business card.
Making your logo bigger on the business card
If you're interested in making the logo bigger on the card, when you edit the card, it will allow you to increase the size of the logo.