iPhone 3G 3.1.3 how apply the PDF security patch without updating to 4.0.2

i have an iPhone 3G running on 3.1.3, how to apply the PDF security update without having to upgrade to iOS 4.0.2

You can't. Upgrading iPhone software is all or nothing, there is no selective approach.

Unlikley anyone will waste time trying to exploit it now if a patch has been released. Not much need to be concerned.

First and foremost, why I am about to say is just I have read and something that Apple does not support. I would also think they would be offended that somebody did a job they were supposed to do....
Apparently, the same iPhone dev team that is responsible for just about every jailbreak out there figured out a to patch the hole they used for their own jailbreaks. This is especially important due to the fact that not all versions of the iPhone were included in the 4.0.2 updates I.e. First gens. So if you want your PDF holes patched up without having to update to 4.slow then you would do that thing Apple does not support and then use the patch.

Please note, I have not personally done this I am only trying to help out a fellow 3Ger who doesn't want to sacrifice their phone. Ill be waiting to update until they make a 3G specific update that will give us 4.0 luxuries while dealing with the lack of RAM. Thank you in adavance for not flaming nor deleing my post.