Assets in the Media panel can be instantly replaced with another asset. This process will ripple out through the project, replacing all instances of the original asset with the replacement asset. This can be very useful if need to use placeholder images or video to assemble a rough edit, for example, and then replace the images in the edit once the final versions are available.
To replace, on Windows hold the Ctrl+Shift keys or the Alt key and drag the replacement asset onto the original asset in the Media panel. On Mac use the Cmd+Option key combination while dragging.