How to update my iPhone 6 software latest version

To solve your problem is easy.

To update iOS, get a newer iPhone model.

Your iPhone 6 cannot update past iOS 12.5.5.

The hardware is not powerful enough.

Go to Settings > General, then tap Software Update. If you see two software update options available, choose the one that you want to install. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now

Thanks

iPhone 6 is too old, there's no iOS update for an iPhone 6.