Wait till the 18th. Make sure you have a good backup
Over the air updating of iOS needs iOS 5 or later so unless you have iOS 5 or later you will not see updating in setting
The following link explains how to update
This link explains how to update using iTunes
This explains how to transfer purchases to your computer
The article can show you how to update the iPhones iOS:
If you are not running iOS 5 then you wont have the option to update through the iPhones settings and will need to update with the option of iTunes on a PC/Mac as shown in the article above.
Also iOS 7 will be available free on September 18, at this point in time Apple has not announced a specific time so that is all the info available now.