Unfortunately, At&t disabled the 4G/4G LTE switch function which is available on other versions. While, 4G LTE is not always the best choose. Some people want to switch to 3G to save energy. Here is way to do it and I tried on my HTC One X. It works:
go to Settings>Mobile Network>Access Point Namesoriginal link: http://forum.xda-developers.com/showpost.php?p=27732183&postcount=8
Click the Menu button, and select "New APN"
Enter the following:
Name: Media Net (name doesn't really matter, as long as its different from the original APN)
Proxy: Not set
Port: Not set
Username: WAP@CINGULAR.COMPassword: Not set
Server: Not set
MMSC: http://mmsc.cingular.comMMS proxy: wireless.cingular.com
MMS Port: 80
MMS Protocol: WAP 2.0
Authentication type: Not set
APN type: default,supl,mms (no spaces after each comma)
After entering the values, hit Menu again, and select "Save". Now go back, and you should see your "new" APN listed along with the "original" one (should be AT&T PTA). Just select the new APN, and hit back or home. You will see the 4G LTE icon disappear, and you will lose data for a minute or so (if you use the browser, it will give a no connection error). Then you should see the "4G" icon come up (no LTE), and you should have your data connection back.