Please help me can’t find my software update on my iPhone

To update your iPhone 7 to iOS 15.3.1

Make sure:

• your iPhone is connected to a charger and is more than 50% charged
• your iPhone is connected to the Internet using Wi-Fi, and not using cellular data or Personal Hotspot
• your iPhone has 6 GB free storage
• your iPhone is not on Low Power Mode

Then go to Settings > General > Software Update > and update from here if you haven't

I have tried that but it still did not work

If you are using a managed device they may not allow you to update on your own and have removed that option.