Apple announced the launch of iOS 4.3 at the press conference last week and said it would be available March 11. However, Apple let the update out a few days in advance, making it available to the public today. All you need to do is plug you iPhone, iPad, or iPod Touch into iTunes and click Update.
Please note, if you upgrade to iOS 4.3 you will lose your jailbreak/unlock. There will probably be a jailbreak available in the next few weeks.