How can I install iOS 16 on my iPhone?

iOS 16 hasn't been signed in over 2 years, so you can't download that to an iPhone 12. iOS 17 isn't signed anymore either. That leaves iOS 18.1 as the current iOS version you can download. If you haven't updated your iPhone past iOS 15, this means you've missed at least a couple years worth of updates, so updating over the air becomes virtually impossible. Instead, connect your iPhone to your computer and update it using iTunes (Windows) or Finder (Mac).

These two links explains how to to do that:

Update software on iPhone, iPad, or iPod touch in iTunes on PC - Apple Support

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Hello~ Click on below and review…

If your iPhone or iPad won't update - Apple Support

As *lobsterghost1* points out …you’re way behind in updating and a computer will be your best option.

~Katana-San~

EvitaBenedicte wrote:

Thanks a lot for your advice! Actually iOS 17 showed up as an alternative, but I couldn’t install this one either.

Then what error message do you get when you try to update? You may need more memory than your phone currently has available. You need to be connected to Wiif. Your phone needs to be charged higher than 50%. The more information you give us, the better we can help you.

EDIT: If you were actually offered, iOS 17, it likely would be 17.7.1 and if you swipe up from there, you'll likely see iOS 18.1 as an option.