How do I install iOS 16.0 on my iPhone

An iPhone 5s, iPhone 6, or iPhone 6 Plus isn't capable of 13 or above; they require at least 2GB of RAM. Try downloading the applications to a newer unit on the same Apple ID first.

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The iPhone 6 can be updated to iOS 12.5.7, no further.

If this is not the issue, please provide relevant details.

You need a new phone. That is the only way you will be able to update the software. That phone does not have the hardware to be able to run iOS 16.

You need to have an iPhone 8 to download and install iOS 16.