Update My Software in 15 ios IPHONE 6 plus

salmanayaz ~ 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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Hello ~ Your iPhone is not capable of being updated and you need a newer device. Go to upper left corner and click on the two lines and then iPhone to see your new choices. If you consider a second hand device please take a moment to read these…

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~

The iPhone 6plus cannot update further then iOS 12.5.5.5 due to hardware limitations