You need ipodwizard which you can download here
Once you have IpodWizard open; change Edit Mode at the top left corner of IPW to Firmware File and click Open Firmware
Find the Firmware that you want on your ipod and click Write to ipod
There are a few firmwares in this section
make sure that you get a 5.5g firmware if you have a 5.5g ipod. The 5.5g ipod has a search feature at the bottom of Music menu and the 5g does not