How do I get the new 15.7 update?

The fact that you're still on iOS 12 may indicate you have an iPhone 6 plus and not a 6S plus.

Go to Settings > General > About and look at your model.

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.6

• If you have an iPhone 6S through 7, you can update to 15.7

• If you have an iPhone 8 or higher, you can update to 16.0.1

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

Update using your computer -

Update iOS on iPhone - Apple Support