Step 1: Select the Edit option and proceed to the Media section
Step 2: Click on "Generative media" to see all the generated images/videos, which you can choose to use in your current videos.
Step 3: Select the image or video you want to replace. Then, hover over the media in the library, and you'll see the option to replace it.
Once you're finished, click on "Proceed" and "Save changes". You can then preview the video!