Carrier settings updates are small files that can include updates from Apple and your carrier to carrier-related settings, such as network, calling, cellular data, messaging, personal hotspot, and voicemail settings. You may receive notifications from time to time to install new carrier-settings updates.
If you see the prompt for this update go ahead and accept it. If you do select not now, you can follow these instructions to update your carrier settings:

Make sure your device is connected to the Internet.

Tap Settings > General > About. If an update is available, a prompt will appear.

Note: Occasionally a carrier may release a mandatory update. This type of update will display only an OK button instead of an Update button, to let you know that the update was downloaded and installed.

If you need to know the version of the carrier settings your device has, tap Settings > General > About and look next to “Carrier”.