Assuming you already have BlackBerry Desktop Manager installed on your pc, the first thing to do is delete any other OS version you have previously downloaded. Then get OS5 for the 9700 here:
http://www.blackberryforums.com/gene...a-english.html
Follow the instructions in the FAQ for downgrading your OS:
How do I downgrade the OS on my BlackBerry? - BlackBerryFAQ
Get OS5 for the 9700 from the link above rather than from the link in the FAQ as the FAQ link probably only has the newest version of the OS.