How to jailbreak iOS?

iOS jailbreaking is a process that allows devices running Apple's iOS (also known as iPhone OS prior to iOS 4.0) operating system (such as the iPad, iPhone, iPod Touch, and recently Apple TV) to gain full access (root access) to unlock all features of the said operating system, thereby removing limitations imposed by Apple. Once jailbroken, iOS users are able to download additional applications, extensions and themes that are unavailable through the official Apple App Store, via installers such as Cydia, one of a number of current means for older iPhones.

Under the DMCA of 2010, jailbreaking is legal in the United States, although Apple may claim it voids your device's warranty.

The jailbreaking process, however, can be quickly and easily reversed by restoring the operating system through iTunes.

Follow the below guide for step by step instructions to convert your tethered jailbreak to untethered jailbreak on iOS 4.2.1:

1. Your device must be on iOS 4.2.1, then download GreenPois0n RC5 for Mac OS X, Windows, Linux  from here.
2. Launch GreenPoison RC5 then connect your iPhone into your Mac or Windows via USB and click on Jailbreak as shown below: 
3. Follow the instructions shown on the screen to enter DFU mode:
4. Once jailbroken, you'll get Cydia installed on your iOS device and you won't need to use Just tethered boot right now option anymore.

                                                            Video guidance  given below.

Step 2

Step 3