I have a Iphone 3GS with a firmware 06.15.00 and I need to downgrade it to enable me factory unlock it
I'm sorry, but Apple does not provide a downgrade path for iOS. Because downgrading is unsupported by Apple we cannot discuss it on these forums, but you can use Google to find information on how to install an older version of iOS on any iDevice.
Factory unlock (via carrier) should work with any iOS version. It does not need a downgrade.
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