You can upload a logo by visiting the settings page. Access the settings page by clicking on the cog icon in the navigation menu on the left or using the drop down menu by clicking on your name in the top right corner. (You may also click here.)
From the settings page, find the "Logo" block. Here you can upload a custom image (with a max file size of 40kb) by choosing it from your computer and clicking the "Upload Logo" button.