Navigate to the Account Settings by mousing over your organization's name in the upper right corner, selecting Account, then click on Settings.
From the Account Setting page, you can remove a logo by clicking on the "X" in the top left hand corner of the image. Once removed, be sure to upload a new logo. Once this change has been made, it will apply to all pages that currently utilize the logo in the header. Older versions of the logo are saved in your media library in the Logos folder.
