help! i have an iphone 3g 3.1.3 and wanted to update it to 4.0 but itunes...

...only allows 4.1 so i downloaded it? from a website that says it from the official website when it came out and now the file is on my desktop, now what do i do and how do i update it using that file please step by step. BTW i want 4.0 and not 4.1 because i want to jailbreak it and i cant jailbreak past 4.0
 
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