Upgrade 3gs 3.1.2 to 6.1.2

Hi, ive searched your tutorials section and couldnt find an answer specific to my question. i have a jailbroken iphone 3gs still running the 3.1.2 firmware it came with. i recently heard you can jailbreak 6.1.3, so i downloaded 6.1.3 on itunes but did-not-install it until i got more info.. i then heard that the jailbreak for 613 is teathered and that it would be easier to deal with 6.1.2. i have downloaded 6.1.2 from iclarified.com, but i have no idea where to put the file or what to do with it, i wanted to let itunes do the update but since 6.1.3 is waiting there in itunes i didnt want to mess things up. i presume ill need to delete 6.1.3 from where ever itunes has it saved and replace it with 6.1.2? if there is a tutorial on this that i missed, then id even be happy if you just sent me the link, otherwise any guidance on what to do next would be greatly appreciated, my 3gs is borderline useless as is. kind regards, daniel capric. vox.columbia@gmail.com