How to update my iPhone 6?

Assuming you have a compatible device, 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. 

These are the instructions to update your iPhone:--> Update your iPhone, iPad, or iPod touch, bearing in mind that a model 6 or 6+ cannot support iOS versions beyond 12.5.6 because of hardware (CPU/RAM) incompatibility.

Regards

Giulio

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