Please update my iPhone 6

Hello,

Here are two different ways to ensure the highest iOS that your device will support is installed on your device.

For instructions about updating your iPhone, iPad, or iPod touch wirelessly, click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Your device will update to the highest version currently available that it will support. With 12.5.7 there is no next update possible.

iOS 12.5.7 is the latest update for the iPhone 6. If you are saying that you already have this, there are no more updates available.

Settings > General > Software Update.