I want ios 15 on iPhone 5c

The 5c is way too feeble to run anything higher than iOS 10. It is almost obsolete as a smart phone and will only really work properly as a phone and texting device.

FaizanAhmed ~ Apple offers many newer iPhone models which will run iOS 15 and the latest apps:

iOS 15 and iPadOS 15 are compatible with these devices

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

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