The logo can be changed under Appearance → Customize → Theme Options → Logo & Site Icon
Recommend height for the regular logo is 90 pixels.
If you want to use a retina logo make sure the logo image is exactly 2x the size of the regular logo. Example; if your regular logo is 180 x 90 pixels in size the retina logo should be 360 x 180 pixels in size.
The favicon can be changed via Appearance → Customize → Theme Options → Logo & Site Icon at the “Site Icon” setting.