How do you update the phone

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. 

eavenh ~ Your iPhone 6 Plus cannot be updated beyond iOS 12:

iOS 15 and iPadOS 15 are compatible with these devices

...found by searching Apple's Knowledge Base.

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