IOS 9 Iphone 5 data not working due to update, What can I do to fix it?

AT&T Sim card with straight talk an iPhone 5 and recently updated to iOS 9.0. I can't get my cellular data to work, the only ones that work are the ones that I allowed before I updated.

Known problem. There's a number of "fixes", but this update may work as well.

Shut the phone off for a couple of minutes then put it back on… It should work

How do I go back to a pervious version. I updated my iphone 5C to 9.2 and my battery is draining and I do mot like the new update and want the old verison back I believe ot was 9.0 version. How can this be done?