How do i put iOS 3.1 on Ipod 1st Generation?

Hi

I know I posed this question yesterday and someone was kind enough to post a link but I couldnt see how to reply to that thread. The links dont work on my computer so I have to open itunes normally and then Im back to the original problem - if i search for iOS 3.1 I only see podcasts. I'm still none the wiser how to do it.

Any help appreciated

Simon