My phone settings is not bringing out download iOS

To update to the maximum of iOS 12.5.7, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

To update that iPhone, select Settings > General > Software Update. If your phone is eligible for an update, you will be prompted to do so. Your iPhone 6 Plus can only be updated to 12.5.6 so it may not be eligible for an update. It is an old phone and the hardware will not support a higher iOS.

You can read more about updates in the link below:

Update your iPhone or iPad - Apple Support