Video Cropping

The video recording starts with a motion like front door or security camera. When the object leaves, the video is still recording. Mine is set to 1or 2 minutes. When I want to save or share, I want to crop the video to show only the beginning of it. The rest is not useful and file size is bigger than I need. I tried to edit in Photos on my tablet after downloading it, but it doesn't let me edit. If I could do it in Ring's app, I don't need to download to take up Google's storage. It's should be a standard feature in Ring app. Thanks.