The footer logo is the logo which appears in the bottom footer section of your website.
To change/edit it
In the left hand menu, go to the Settings > Configuration > Media (the media section allows you to essentially upload any logo to your website and simply select the drop down option underneath to specify how you want to use the logo)
You will see a series of logos that have been uploaded to the media section. You will see one that has a label of Footer Logo applied to it from the drop down option at the bottom of the image
To change the footer logo. Delete the existing logo by clicking the red bin icon in the top right hand corner of the logo you want to remove
Upload your new logo by dragging and dropping your file into the media section, or click the media section to choose which file to upload
Once uploaded, click the drop down box underneath the new file you have uploaded and select Footer Logo from the drop down options
Your changes will automatically be saved
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