How to update my iPhone 6 iOS 12.5.7

Go to Settings/General/Software Update and follow the guidance there. If you encounter a problem post back with a description of what went wrong.

Connect to Wi-Fi and charger and go to:

Settings > General > Software Update.

If a new iOS version is ready for download it will appear.

You can see how to update your iPhone here:

Update your iPhone or iPad - Apple Support