This video walks through this process:
1. Launch with the Ads Launcher
2. Make adjustments in Facebook Ads Manager.
In the video you'll see how to customise the 'Story' and 'Reel's placement by uploading a different size video.
If, however, you need to replace the ENTIRE media because you don't want to use the template media, then just delete the media first when editing each ad and then upload your new image or videos.
If you need additional help connecting your Facebook Account to the new Ads Laucher tool please follow these instructions: https://help.ziplineplatform.com/articles/220715-ads-laucher-set-up