Just replace the image in your Webflow designer in the correct format. The device is vector-based and sharp at any size. The screen image will adjust to the device on a percentage basis.
Pro tip: the best way is to make the iPhone a Webflow symbol (component) so you can easily replace the image via the element settings.
You have to change the shadow values in the embed element. If you want to use shadows you still can use Webflow symbols (see iPhone example below).