How can I update my iPhone 6

To update to the maximum of iOS 12.5.7, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

"How can I update my iPhone 6"

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Updating an iPhone:

Use a Computer to Update:

Use my User Tip:  Update your iPhone Using a Computer - User Tip. If any updates are available, they'll install, sure enough.