Q: How do I install carrier settings on a 3GS?

Hi,

I recieved an email from Amaysim (otherwise known as Optus) telling me I need to change the APN on my phone to Internet. When I opened network I dont seem to have the data settings inorder to change it. I googled and found out I could fix it by installing the carrier settings on itunes. How do I do it? I have tried everything and I still dont have the menu on my phone. Please help!