First, you MUST restore your iPhone on iTunes in order to prevent your iPhone from going into a reboot loop, because updating to official updates from jailbroken updates will "brick" your iPhone (although this can almost always be fixed by recovering it in iTunes) THEN you must download the latest jailbreak tool (I.E. Redsnow/Absinthe) and jailbreak it by following the instructions on the tool.
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First, you MUST restore your iPhone on iTunes in order to prevent your iPhone from going into a reboot loop, because updating to official updates from jailbroken updates will "brick" your iPhone (although this can almost always be fixed by recovering it in iTunes) THEN you must download the latest jailbreak tool (I.E. Redsnow/Absinthe) and jailbreak it by following the instructions on the tool.