Software update is not showing

Hi, if you are going to update your phone you need to follow these steps:

Update your iPhone, iPad, or iPod touch wirelessly

1. Back up your device using iCloud or your computer(not necessary)
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see two software update options available, choose the one that you want to install.
5. 
6. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.

You might see that your device is up to date and that there's an upgrade available. For example, even if your device is up to date with the latest version of iOS 14, there might be an option to upgrade to iOS 15. You might want to upgrade iOS when you pair an accessory with iPhone.

If you need more space when updating wirelessly

If a message asks to temporarily remove apps because the software needs more space for the update, tap Continue to allow apps to be removed. After installation is complete, those apps are automatically reinstalled. If you tap Cancel instead, you can delete content manually from your device to add more space.

Obtained from: Update your iPhone, iPad, or iPod touch - Apple Support

Hope this answer will help you