Access Appearance / Theme Options / Header / Header Site Logo and upload your logo image into Site Logo Image field.
You will also need to upload file for Site Logo Image for Retina Screens field, that will help your logo image not become blur on high resolution screens.
Remember configure Logo Width and Height and also Logo Padding option until you see the display on your site is perfect as you want.
If you want to live preview in changing things, you can access Appearance / Customize / Header / Header Site Logo instead.
