Navigate to Dashboard>Appearance>Customize>Site Identity. Select the “Select Logo” button and upload your logo.
For best results, your logo height should not be more than 100px (200px for Retina) high. It is possible to add a logo that is taller but it starts making the header too big and just doesn’t look good visually.
If you remove the logo and the header doesn’t look right, hit publish and refresh the page. It’ll look fine after a quick refresh.