How can I update my iPhone 6s ?
Was my iPhone 6s original or fake ?
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Your topic and question are completely different.
For question:
See if it appears under warranty information:
https://checkcoverage.apple.com/
or:
Settings > General > About ( if using iOS 12.2+)
For topic:
Click on this link to read how to "Update your iPhone, iPad, or iPod touch" --> https://support.apple.com/HT204204 - When you try to update you will only be given one update selection which will be the highest available iOS version supported by your device. If it says no update is available and you need a higher version you will have to buy newer equipment.