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Two ways that need no extra app: the Adjust sliders in Photos, or this page in Safari for highlight recovery and a before and after split.
To darken a video on iPhone, open it in Photos, tap the Edit button, tap Adjust and drag Exposure to the left. Then lower Highlights if a sky or window is glaring, and raise Black Point if the picture looks grey. Tap Done. The edit applies to the whole clip, and you can undo it any time with Revert to Original.
Photos is quick, but it judges by eye and shows one version at a time. If you want an automatic correction that measures the clip, a split before and after view and a readout of how much of the frame is blown out, open this page in Safari and use the tool at the top. It runs on the phone itself; the video is not uploaded.
Find the clip in Photos, open it and tap the Edit button at the bottom of the screen (it is shown as an icon on recent versions of iOS).
Tap Adjust, then swipe along the row of controls under the video. Tap Exposure and drag its dial to the left until the picture stops glaring.
Tap Highlights and drag it left to calm a bright sky. Tap Black Point and drag it right so the darkest parts look properly dark again.
Tap the video to flick between your edit and the original. Tap Done to save. Photos keeps the original, so Revert to Original undoes it later.
The Adjust row in Photos holds more than a dozen light and colour controls. These are the ones that matter for an over-bright clip, with what each one does to the tones:
Each slider runs from −100 to +100 and starts at zero. Small moves go a long way; two or three controls at around −20 to −40 usually look better than one pushed to the end.
The tool on this page works in Safari on iPhone and iPad. It’s worth using when the Photos sliders aren’t enough, for example when a sky needs pulling back without darkening the people in front of it.
The result is a new video; the original in your library stays as it was. For a deeper look at what the controls do, see how to fix an overexposed video.
(enlarge image) The tool at phone size after Auto fix. Export video stays at the bottom of the screen.
Recent iPhones record HDR video (Dolby Vision) by default. On the iPhone’s own screen these clips are deliberately brighter than everything around them, which is why a clip can look dazzling in Photos and normal when you send it to someone with an older phone or a laptop.
Not permanently. Photos stores the edit alongside the original, so you can open the clip, tap the Edit button and choose Revert to Original at any time. With iCloud Photos the edit appears on your other devices too.
Photos applies adjustments to the whole clip. You can trim the clip first and darken what you keep, or use the trim control in the tool on this page.
It is probably HDR, and the Windows app is showing it without proper tone mapping. Save a standard-range copy with the tool on this page and it will look the same on both.
Highlights. Drag it left. If the sky is pure white it has no detail left, and darkening it will only make it grey, so stop when it starts looking flat.
Photos and iMovie are free and already on most iPhones. You can also use this page in Safari, which needs no app and doesn’t upload the video.