How to update iOS to 16 instead of 17?

If I understand right, you want to download iOS 16, but it only gives you the option to download iOS 17. This happens because your device is compatible with iOS 17 and when updating an iOS device it can only update to the most recently released compatible version.

You cannot, unless your device can only be updated to iOS 16. It will automatically update to the highest iOS it can run.

I need ios16

Then, you need an iPhone 8 or an iPhone X.

Do you have one of these models?

If not, what model of iPhone do you have?