The iOS 12.3.1 update is a small maintenance upgrade, and it shouldn’t take too long download and install on your iPhone, iPad, or iPod touch.

Apple’s iOS 12.3.1 update comes with a short list of bug fixes for the iPhone, iPad, and iPod touch.

If you’re moving your device from iOS 12.3 to iOS 12.3.1 you’ll get the shortest change log and the smallest download. The iOS 12.3.1 update is 80MB for the iPhone X. It’s around the same size for other iPhone and iPad models currently running iOS 12.3.
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