What's the latest iOS update available for iPhone 5?

Hello,

Here are two different ways to ensure the highest iOS that your device will support is on your device.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Your iPhone 5 cannot be updated beyond iOS 10.3.4. It does not have the hardware needed to run anything higher.