Adding your own logos and favicons in our themes is really easy.
Head over to Appearance > Customize > General Theme Options > Site Identity. There, you will find the Custom Logo and Custom Favicon fields. To add your own images, all you have to do is upload them there and save.
Be aware that some free themes may not have the option to upload a favicon. If there’s no Custom Favicon field to be located in the Customizer, that may be the case.
For logos, usually the recommended image dimensions are little above 200 pixels wide, and around 100 pixels tall at most. This may vary depending on the theme and its layout. Also, it is strongly recommended to use images that either have a transparent background, or that blend with the background color of the layout; doing so will surely improve the professional look of your site.