If you mean in the Gallery, you can go into the Gallery settings and disable Super HDR. If you mean while watching content , unless you go into Power Saving mode, there is no way as HDR is triggered by the content itself rather than the phone. The content you watch have HDR parameters embedded in them that they transmit to the screen (like color and light map mapping). When it goes into power saving mode, the phone ignores some values , limiting them and forcing the content to be displayed normaly. Though, you can just...lower the brightness..you know.
