Go to www.ziphone.org and download the required files here currently available for windows and mac. Follow the instructions on the video below:

ZiPhone2.4
First! Copy or Move the ZiPhone2.4 folder to the C:\ folder (to make it easy, and have it named to ZiPhone)
Plug in your iPhone
Windows + R (RUN): cmd
Command Prompt: cd c:\ziphone [ENTER] (Should say folder’s destination in cmd)
In cmd type: ziphone -e (NOT for 1.1.1 or prior (3.9BL OTB), only for those who have already jailbroken or unlocked their phones with 1.1.2 or later, and not needed for those who have 1.1.2 or 1.1.3 OTB)
Once Restarted, Restore to 1.1.3 (If you get ERROR 6 Restore again)
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Key:
A: To Unlock, Jailbreak, & Activate (for any GSM, use for any 1.1.2 or 1.1.3 OTB iPhone)
B: Jailbreak (for AT&T users who registered through iTunes)
C: Jailbreak & Activate (for AT&T users using their OLD sim OR if you have ALREADY UNLOCKED your phone using ZiPhone)
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In cmd type:
A: ziphone -Z Y
B: ziphone -j
C: ziphone -Z N (less)