iOS update is not showing in my iPhone settings.

The red badge is telling you that you have an update or something that needs attention. If you tap on Settings > General > About > Version it will tell you what iOS version you are currently running.

If you tap on General on your screen it will show Software Update whether you have done so or not. If you have updated and you tap in Software Update it will tell your that you are already updated if you are running 12.5.6 otherwise it will ask if you want to update.

To update your iPhone you will need to have a 50% battery charge or better and turn off Low Power Model. You will also need around 6GB of free space.

Hello,

It should be listed under "About". You won't see "Software Update" because you've already received it. See this for issues:

Updating iOS: common issues and solutions

Good luck.