Why my iPhone is not showing update option

The message on your iPhone is telling you exactly what is required for the download to occur:

Connect the phone to a good Wi-Fi connection.......(not hotspot)......then try the update again

snehu143 Said:

"Why my iPhone is not showing update option"

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Thank you for the screenshot.

Update Using a Computer:

Alternatively to using WiFi, you update using Ethernet through a computer. See my User Tip: Update your iPhone Using a Computer - User Tip. On the computer, you would connect it through Ethernet, going directly into your router. For a Mac (or PC) that does not have Ethernet, this would be the adapter to use: Belkin USB-C to Gigabit Ethernet Adapter - Apple