Can I update an iPhone 7 from iOS 15.8.3 to 17?

No, that is NOT possible. iOS 15.8.3 is as far as an iPhone 7 can go. It doesn't have the hardware needed to run iOS 17 or now iOS 18. If you need a higher level iOS version than your phone can support, then you'll have to buy a newer, more modern iPhone.

Hello~ No…you need a newer model iPhone. Click on below…

iOS 16 and iPadOS 16 are compatible with these devices - Apple Support

~Katana-San~

Can I upgrade my iPhone 7 Plus to iOS 16

No

You need an iPhone 8 or iPhone X to run iOS 16.

An unit on 15.8.3 isn't capable of 16 or above.

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Can I upgrade my iPhone 7 Plus to iOS 16

Already been answered.