How do you upgrade iOS operating software on a 4.2 iphone to at least iOS 4.3?

If your iDevice is using a version of iOS lower than 5, you will need to use iTunes on your syncing computer to perform the upgrade. Use the Apple link below as a guide for the upgrade.

http://support.apple.com/kb/HT4972

Also read the instructions from the section entitled "Update your device using iTunes" at the link below.

http://support.apple.com/kb/HT4623

Information regarding transferring purchases from your iDevice to iTunes on your syncing computer can be found at the link below.

http://support.apple.com/kb/ht1848

Follow this article to upgrade through iTunes: http://support.apple.com/kb/HT4972

It will automatically download the newest version of iOS that your iPhone supports, which, if you have an iPhone 4, is iOS 6.1.3.

If you have an iPhone 3G, it can't be upgraded past iOS 4.2.1.