If your carrier pushes a carrier settings update, the iPhone will automatically prompt to install the update. Tap Update when the dialog box appears.
You can also check for a carrier settings update manually. Follow these steps to see if a new carrier settings file is available:
1. Make sure the iPhone is connected to cellular data or Wi-Fi
2. Navigate to Settings -> General -> About
3. Wait a few seconds for a dialog box to appear
4. If nothing happens, there is no carrier settings update available