How can I install ios15 on iPhone 6

An iPhone 6 cannot update past iOS 12.5.5. You need a new phone.

You can not, as it is not compatible with iOS 13 or higher.

Adepoju ~ Apple offers many iPhone models which will run iOS 15:

iOS 15 and iPadOS 15 are compatible with these devices

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

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