Your header is set via 'Page Header Settings' within your page. If not, go to the how-to for setting header via WPBakery Page Builder.
1. To change your header, you must first remove it. To do so, click "Remove Upload.
2. Then click on 'Upload'. From your Media Library you can choose the new image for your header. If it is not there yet, click on the 'Upload files' tab to add your image to your Media Library.
Notice:
Your new image must be between 1600 and 2000 px wide, and at least 475 px high. If you get your material externally, ask your designer about it.
