To reverse a video on an iPhone without installing anything, open this page in Safari, tap Choose a video, pick the clip from Photo Library, and it plays backwards straight away. Tap Export reversed video, then Share or save to Photos and Save Video. The reversed copy lands in your Recents album next to the original, which isn’t changed.
Apple’s own apps don’t do this. Photos can trim, crop and filter a video, and turn a Live Photo into a Bounce, but it has no reverse. iMovie for iPhone has a speed control but no reverse switch either; only iMovie on a Mac has one. So the realistic choices are a browser tool like this one, or a third-party editor such as CapCut, both covered below.