How to slow down or speed up a video
This changes the saved file itself, not just how a player plays it back. Here is what actually works, including keeping the audio in tune.
Playback speed versus the actual file
Many video players, including YouTube and some phone players, let you change playback speed while watching, but that setting only affects how that player shows the video to you; the file itself, and how it plays for anyone else or on any other app, is unchanged. Actually speeding up or slowing down a video means re-encoding a new file at the different speed.
On an iPhone
The Photos app has no built-in speed control for existing videos. iMovie (free on iPhone) can do it: add the clip to a project, select it, and use the speed slider to make it faster or slower, then export. Apple's own Slo-Mo camera mode only works for footage recorded that way from the start, not for slowing down existing regular video.
On Android
Most Android phones' default Gallery or Photos apps don't include a speed editor. Google Photos' basic editor also doesn't offer this; changing speed typically means a separate video editor app.
On a Mac or Windows
iMovie on Mac works the same way as on iPhone: select the clip, drag the speed slider, export. Windows' Clipchamp (built into recent versions of Windows) includes a similar speed control on its timeline. Both are full editors for what is otherwise a single adjustment.
A direct speed changer with pitch correction
Stayput's change video speed tool speeds a video up to 2x, 4x or 8x, or slows it to 0.5x or 0.25x, directly in the browser, and keeps the audio's pitch correct rather than letting it sound chipmunk-high or slowed to a growl, which is a common side effect of naive speed changes. Nothing is uploaded to do it.
Questions
Does changing video speed also change the audio pitch?
It can, unless the tool specifically corrects for it; naive speed changes raise the pitch when sped up and lower it when slowed down, while pitch-correct tools keep the voice or music sounding natural.
Is changing playback speed in a video player the same as actually speeding up the file?
No, a player's speed control only affects how that app plays the video back; the saved file, and how it plays anywhere else, stays at its original speed.
Can I slow down only part of a video, not the whole thing?
Trim the clip to just the section you want first, then apply the speed change to that shorter clip, since most tools change the speed of the whole file by default.
Why would I want to slow down a video that wasn't recorded in slow motion?
Slowing down regular footage can create a slow-motion effect after the fact, useful when the moment wasn't recorded with a slow-motion camera setting to begin with, though it looks less smooth than footage recorded at a genuinely higher frame rate.