To access the Customizer via the WordPress Admin Panel, click on Appearance and then Customize.
To access the Customizer from the front end of your site, again, when logged in as an administrator, click on the Customize link in the WordPress Admin Bar.
Once the Customizer has loaded you will see a Sidebar on the left with some tabs, click Site identity.
The Site Identity options allow you to add a logo, edit the site title and add a Site Icon/Favicon.
To add a logo just click the Select logo button and then drag and drop or select your logo from the media library. The same instructions apply to the Site icon.
Be sure to add an appropriately sized logo and on mobile devices the logo height is capped to 40px height. This ensures large logos do not take up too much space on mobile devices.