why can’t I update my phone?

Hey there,

What happened when you try to update your iPhone?

and what do you see when you go to Settings > General > Software update?

what is your iPhone model?

See>>>Update your iPhone, iPad, or iPod touch - Apple Support

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if you have trouble with update See>>>If your iPhone, iPad, or iPod touch won't update - Apple Support

if you have iPhone 6 or less you can’t update it to newer version the last update for this device is iOS 12.5.5