If you already did the company set-up you can always change this whenever you want. It will not affect already published and draft campaigns, only new ones.
Go to 'Profile & Settings' in the righthand top corner. Click on this link and the sub menu's will be displayed.
Go to Account Settings
The next screen will appear. Go to 'Upload a new logo' to do the change. You can upload the logo as JPG, PNG or GIF. You can also drag and drop a new logo into the specific field.