Steve Jobs announced iOS4 for developers at WWDC 2010.Apple iOS 4 aka “iPhone OS” is all set to be released on 21st June for all the prior iPhone and iPod devices while iPhone 4 will be released on 24th June.
Features of iOS 4 includes multitasking,folders,enhanced mail,deeper enterprise support and Apple’s iAd mobile platform.
To install iOS4 in your old iPhone you need to iTunes 9.2 or later.
Video which will show you how to install iOS4:
Download iOS 4 .