In the premium versions, if You import the demo content, sometimes it is hard to find, how to change the demo logo.

The logo on the sticky header is different than the main logo. You can change it in the customizer’s Layout-> Primary navigation-> Navigation logo panel. In some newer themes this panel is in the Site identity section.

Unfortunately sometimes after the demo import, in the customizer it looks like that panel is empty. In this case add an image to the panel, than remove it, than the demo logo will disappear.