Apple's massive overhaul to its iPhone and iPad operating system, iOS 7, is now available. If your eligible device is reasonably up to date already, you can just click Settings -> General -> Software Update to download it straight to your phone, but be careful! It's a major change from the iOS you know, so it pays to learn all about it first.
If you do nothing else, click "Learn More" when it tells you that iOS 7.0 is available (see screenshot). You'll see a lengthy list of stuff worth skimming before you commit.
For eligible devices and further cautions, have a look at my previous story. And for a lot more info, start reading this massive review done by our friends at Laptop.