To change the control panel logo, login to your administrator control panel and ensure that you're logged in to the Operations platform. You will know if you are by looking in the top right corner - you should see the following links.
From this system, select Settings.
Select Brands (n.b. if you don't see this screen then you are in the Billing system rather than the Operations platform - click the Operations link in the top right corner and select Settings from the left menu).
Select your control panel URL from the list.
On the General tab, click the Edit button.
Upload your new logo using the Choose file button.