You can upload your logo image by navigating to the Appearance->Customize->Site Identity. If the logo is not uploaded, then your site name will be used.
Please, follow the steps below to upload your logo:
Step 1 – Login to your WordPress Administration Panel (Dashboard).
Step 2 – Go to Appearance->Customize->Site Identity.
Step 3 – Upload a Logo. Size doesn’t matter but if you want to make logo retina use a big size (Our logo is 80px x 43px).
Step 4 – Set the width size for your logo (default 300).
Step 5 – Upload your favicon – should be a 512px x 512px png image.