Click on the customization option on the top bar, a left sidebar will appear with customizable options, you will see an edit icon on the logo, click on that edit icon and change the logo text.  

If you want your logo as an image, you need to enable the Image logo required. To enable, go back to the customization and click site identity > select logo > upload your logo image. Here, you can crop the image or skip cropping. Now your image logo is ready. Click on the Publish button to save your changes.