how i download ios 12 for iphone6

You can only update to the latest iOS version your iPhone support and nothing in between.

iOS versions available for iPhone models below:

iPhone 5s, iPhone 6 and iPhone 6 Plus => iOS 12.5.5

iPhone 6s to the latest iPhone 13 Pro Max => 15.1

Check your model in Settings > General > About > Model Name > here ?

FYI: iOS 12 is not available anymore.

You can only update to iOS 12.5.5 on your iPhone 6.

touch261 ~ Learn how to update your iPhone, iPad, or iPod touch to the latest version of iOS or iPadOS:

Update your iPhone, iPad, or iPod touch

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

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OK - so same question, which. What's the process to update an iPhone 6s to 12.5.5? I'm on version 11.3 and it won't update to iOS v15.