To view the APN settings:
You can view the APN settings on your iOS device in the following location if it's permitted by your carrier:
iPhone: Settings > General > Cellular > Cellular Data Network
Tap each field and enter the APN settings provided by your carrier. The settings are saved automatically.
Source:Apple Support Communities
Like Andrew Lima said, if you want to change them however, you must contact your carrier, because every carrier is different.