Only Global Admins can make this change. From the Home Page of the Adminstration Dashboard > Click the "Administration" section. > then Click the "Site Settings" icon. Under the Organization header you will see a browse button called "Upload Ogranization Logo" click this button and upload your company logo.
Note: The logo appears in the upper left hand corner of the screens header. Logos must be sized to 370 x 80 pixels using a jpg or png format.