I’m unable to update my iphone 6s, my current ios is 11.4 , I want to update to ios 12 or 13

Hi,

To update your iPhone, go to settings/general/software update.

Make sure you’re connected to a stable and fast internet connection (not cellular connection, only wifi)

If you can’t update your iPhone, restart you iPhone and try again.

You can also update your iPhone through iTunes.

Try updating it using itunes on your computer-

Using iTunes, you can update software on your iPhone, iPad, or iPod.

1. Connect your device to your computer. ... 
2. In the iTunes app on your PC, click the Device button near the top left of the iTunes window.
3. Click Summary.
4. Click Check for Update.
5. To install an available update, click Update.

If it is iPhone 6S then you can only update to iOS 14.5.1