It sounds like it is saved in the slide master. This should work (although I am by far not a PowerPoint expert).
In the ribbon click on the "View" tab, and then click on Slide Master
The left hand side preview should update, and you can now click on the first slide (which is the master slide). Right-click on the first slide and select Master Layout. Select the footer checkbox, and click ok.
The footer should now be accessible to you in the master slide for updating, which when updated will be inherited by the rest of your slides. Once done editing, click on 'Close Master View' in the ribbon, and that should be it.