Logo
The logo can be changed under Appearance → Customize → Logo & Favicon

Retina Logo
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.
Favicon
The favicon can be changed via Appearance → Customize → Logo & Favicon at the “Site Icon” setting.