iOS update

It is not possible to install iOS 13 on any iPhone. If you have an iPhone 5S or 6 the latest you can install is iOS 12.5.2. If you have an 6S or newer you can install iOS 14.4.2. But you can’t install any version other than these.

The one exception is if you have a 6S or later and you participated in the iOS 13 beta you can continue maintaining it, but you shouldn’t. You should remove the beta and update to iOS 14→How to uninstall iOS beta software - Apple Support

Maybe stuck on a boot loop

if so here is the support from Apple when your iPhone is stuck on a boot loop

https://support.apple.com/en-us/HT201412

and iPhone 6 software last update is iOS 12 so iPhone 6 won’t get the iOS 13 or 14

only iPhone 6S has iOS 13 or 14

Start with this support article→If your iPhone is stuck on the Apple logo - Apple Support

EugeneAskWhy, please read a thread before answering it, rather than repeating what has already been posted. There are plenty of threads that can benefit from your expertise that have not been adequately answered. Your replies aren’t wrong, they are just redundant.

Dice ~ If your iPhone 6 still has the original battery, it probably needs replacing:

iPhone Battery & Power Repair

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Does not network