First, if you haven't already, determine if your logo will be text-based or image-based.
For image based logos:
Upload your logo image file to your media library. Then double click the existing logo image in order to replace it with your own. If there is not an existing logo image in your design, simply click to select the canvas you want to put your logo on. Then click the media library icon (bottom middle of Showit, 3 icons, right side icon) and select your logo image and add it to the canvas.
For text based logos:
If there isn't an image-logo in your site design there is probably a text-logo. Just double click the text logo already in your design and change the text to your business name. You can then choose to tweak the font to find the perfect one for you!