Why is sofwate update option not showing on my iPhone?

You have not told us which iPhone you have.

I'm guessing that your iPhone is not offering you any updates, and the reason is that it is already running the highest version of iOS compatible with it.

• You can update an iPhone 5s, 6, or 6 Plus to the latest release of iOS 12. You cannot update any of these phones to iOS 13 or higher.
• You can update an iPhone 6s, 6s Plus, SE (1st generation), 7, or 7 Plus to the latest release of iOS 15. You cannot update any of these phones to iOS 16 or higher.
• You can update an iPhone 8, 8 Plus, or X to the latest release of iOS 16. You cannot update any of these phones to iOS 17 or higher.
• You need a more recent phone than any of the above to update to iOS 18. (See: iPhone models compatible with iOS 18 - Apple Support)

Tap Settings > General > Software Update

Does the message say that your software is already up to date? If yes, you have an older phone and there are no updates available for your iPhone.