1) Find the Sat Nav forum
2) Post your question there.
Actually, you have two ways you can update the maps in your Sat Nav.
1) Get a lead (You'll need a USB to Mini USB lead - less than a pound in the right places)
2) Download a map to an SD card
You'll probably need to check the manual on the garmin website
Why am I helping an idiot who can't even post in the right place?