Login into the admin panel.
Go to System > Settings.
Click “Edit”, from “Your Store (default).
Click the “Image” tab.
From there you can replace the logo and the favicon.
The default size for the logo is 268 x 50 px. Just click on clear and then browse, select the image from your computer and upload it.
The favicon can be a .png image, not an icon format (.ico).
A favicon should be an image with the same width and height, for example: 50 x 50 px.