Nikon Support Site lists all the firmware for their cameras. Download v1.0.2 for the D90 using that link.

I believe the steps are as follows:

1. Unzip the file
2. Run the setup routine and follow the instructions
3. When finished navigate to your desktop
4. Locate the Nikon folder
5. Inside this folder copy the NKLD01002.BIN file to the root of the memory card
6. Turn on camera and go to the menu
7. Go to the setup menu (the wrench looking symbol)
8. Select Update
9. Confirm firmware upgrade (e: 1.000 -> 1.002)
10. Click Yes
11. DO NOT TOUCH THE CAMERA while the firmware updates.
12. Message appears saying firmware updated and that camera needs to be turned off
13. Turn camera off
14. Turn camera on and navigate to the firmware menu option
15. Ensure version number is now the new version (ie: 1.002)

NOTE: I have already upgraded mine, so you may actually be prompted at step 3.

Update 1

You can follow the instructions here: Updating Distortion Control Lens Data If you want picture based instructions.