APN Settings can only be changed on devices that allow it by the carrier. The carrier file has to be current from your carrier and they have to allow it. If you do not have the latest carrier file on your device, see this for updating. Update your carrier settings on your iPhone or iPad - Apple Support
Check with your carrier to ensure that you have the latest version. Go to Settings>General>About>Carrier and it will tell you what you have. Then you should be able to adjust your APN settings. If you do not have your carriers file on your device, you cannot make that change.