'Why won't my iPhone update to iOS 13?': How to troubleshoot iPhone updating issues in 4 ways

If your iPhone won't update to iOS 13, it might be because your device isn't compatible, or you don't have enough storage space.
CNET

Didn't Get iOS 26? Your iPhone Might Be Too Old. See the Models That Support It

On your iPhone, go to Settings > General > About and look at Model Name. If your phone isn't on the compatibility list, it will keep receiving iOS 18 security patches for a while, but you won't see ...

Here's how to download iOS 13, Apple's big new iPhone update

It's time to update your iPhone with Apple's latest software update, iOS 13, which began rolling out on Thursday.
PCMag

Make Sure You Really Want iOS 26. There's No Going Back to iOS 18

As it usually does after a new iOS release, Apple has stopped validating its older mobile OS. So, if you upgraded to iOS 26, you better get comforable with Liquid Glass.