How to update iOS 4.3.for iPhone 3GS?

How can I update iOS 4.3. For iPhone 3GS?

Plug your phone into itunes. Then it should download ios 6.1.2

Whenever you choose to UPDATE anything, you need to specify TWO things:

The old version, and, the NEW version. And, regarding the version you listed, is it the old or the new version?

FYI: This is the also the wrong Category for Cell Phone questions, please use the Consumer Electronics > Cell Phones & Plans Category, in future, that's where all the Cell Phone Experts hang out.

TIP: All iPhone Updates are done using your iTunes Account. And, to Update from iOS 4.3, to iOS 6.x just look for the latest iOS Update in the Apps Section on the left Side Bar in iTunes.