It's probably one of the first things you may want to do when you start customizing the theme to align its looks to your brand. To upload your logo, open up the Customizer (Appearance > Customizer) and navigate to the Header section.
Can I upload an alternative logo for mobile devices?
Yes you can do that, you can find this option right below.
Can I upload an alternative logo for the transparent header?
Sure, you can actually set up two different color schemes for the header elements and assign an alternative logo to each one. This give you more flexibility if you're using different header settings in different pages. There's a separate tutorial on setting up a transparent header.