I want to replace my iPhone
It’s battery is defective
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You may not need to replace your whole phone, you can contact Apple for a repair and just replace the battery.
iPhone Battery Replacement – Official Apple Support
Why do you feel that the battery is defective?
Lithium batteries degrade over time, this is why your battery might not last as long now as when you originally purchased your device.
If you go into Battery > Battery Health you will see a percentage that shows how your battery is doing. If it says 90% this means your battery only holds 90% of its' original capacity now.
Apple estimates after around two years it should reach 80%, after that figure it does become a cause for concern.
You can find out more information about battery, charging and optimising your battery to get the most out of it here