How can I change the title of the main product section?

To change the main product section title, just go to the Main product section of the Search box preferences and type your desired value in the Title field:

Note that you can use the tag {query} to show the current text typed by the customer. For instance, if configured as in the image above and the customer is searching with the text "a", the title will show:

Top search result for "a"

Note as well that if an empty value is configured, no title will be displayed.