When you receive it on the other end, tap the video to download it, then save it to your camera roll by tapping the Share button and Save Video. From there, you can upload the video to any third-party service—Instagram included.
This method works, but comes at the expense of compressing your beautiful HD video into something not necessarily as nice. But there's another way.
USE ANOTHER iOS DEVICE: Currently, your iPhone will export slo-mo video to any service listed on its Share sheet. That means you can send an iMessage to yourself, upload video to a Shared Stream, or ping for an AirDrop.
You might think: "Wait, why am I using email if I can just send myself an iMessage or a Shared Stream?" For some reason, Apple has prevented this—send an iMessage and it displays at regular speed on an iPhone 5s (but weirdly, it will display in slo-mo on any other device); send a Shared Stream video, and it displays in slo-mo, but there's no way to download it from the Photos app to your Camera Roll (presumably because iOS assumes you already have the video).
However, you can download videos from either iMessage or Shared Streams on a different iOS device—and you can even AirDrop the exported slo-mo video. As such, if you have an iPad or old iPhone around, this is the time to use it.
Just use the Share sheet to send your slo-mo video to your iPhone, iPod touch, or iPad via iMessage, Shared Streams, or AirDrop. From there, you can post to Instagram or another third-party app from your second device.
But what about my computer?
Yes, you're going to run into the same problem downloading clips to your computer. The good news is that iMovie, Final Cut Pro X, and most other major movie-editing software have time-slowing modes—it just means that you have to import your clips there first.
Climb aboard the slo-mo train
Will Apple create better export tools for Slo-Mo mode in the future? (Will it perhaps let you export your clips to iMovie for iOS?) We can only hope. But in the meantime, these tricks can hopefully get you posting ridiculous slo-mo clips on Instagram in no time.
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