Just lookup your T400 in the Lenovo Support site and download the software. Enter your seven digit model number in the files and follow the instructions.
http://www.lenovo.com/support
Do you have a data plan that allows this?
NO! The normal data plans for the laptops is limited to 5 GB of data per month. Data is not free. You should proceed with caution to reduce the chance of a very large data bill from AT&T.
Dave