You can upload a new logo for your Agency using the Logo field on your Agency Details page.
Click the "Click to Upload" button in the Logo field to open your a file browser on your computer and select your logo to upload it.
Underneath the Logo field, there is a toggle called "Whitelabel Logo?", click this button to toggle the Whitelabel setting on or off.
Once you have uploaded your new logo and have toggled the "Whitelabel Logo?" option to be on, click the "Save" button to save your changes and upload.