I don't see the problem?

1. get winflasher from here and install it

2. open up your N9's sim tray and look for the product code...
3. get the corresponding firmware image from here
(It's the one about 1.1-1.2 gig in size, e.g Dfl61 Harmattan 40.2012.21-3 Pr Legacy 005-oem1-958 Arm.bin)

4.move the image into the Flasher folder where the flasher.exe is located and run a CMD (with admin permissions!!!)

5. in CMD goto the Flasher folder (with flasher.exe and your firmware image) and start flashing using the comprehensive firmware flashing guide...

READ THE GUIDE CAREFULLY!!!
you may want to use flasher like this to replace just the N9's OS and keep your user data