How can I update to iOS 12

Mg ~ Learn how to update your iPhone 5s to iOS 12:

Update your iPhone, iPad, or iPod touch

If your iPhone, iPad or iPod touch won’t update

Updating iOS: common issues and solutions - Apple Community

...found by searching Apple's Knowledge Base.

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Mg ~ What model of iPhone is it?...

Settings > General > About > Model Name

And what version of iOS is it running?...

Settings > General > About > Software Version

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