The logo in the footer is just an image inserted into the widget area in the footer.
To change or remove this logo go to Appearance > Widgets and click on the Footer 1 widget area. You will see a text widget which contains the logo code:
[vc_single_image image="3492" alignment="center" border_color="grey" img_link_target="_self" styling="margin-bottom: 0px;"]
You can replace this code with your own HTML image tag e.g.
<img src="http://domain/com/image-name.png" alt="" />
Alternatively go to Pages > Add New and insert the Single Image element into the page and change the settings to whatever you want for your footer logo. Switch to Classic Mode and click the Text tab and copy and paste this code in the Text widget.