Otherwise an “SD” card and reader will set you back around $30 at walmart, I think the SD card only needs to be 128 megs or so, I use a 256 meg card to do the updates on mine, pretty much you should be able to find an sd card for around $15, and a card reader for $15, you don’t need a big card, you don’t need any name brands. Personally I’d stay at 1 gig or under myself, 128meg 0r 256meg is probably all you’ll ever need.
You can either load the file onto the card yourself, or lowrance makes a nifty little program that put it on the card for you. It’s pretty easy really.
Pick out your unit here http://www.lowrance.com/Downloads/Updates/default.asp attach the card and card reader to the computer, and click on the install now box, after you click on the unit you are updating. Instructions are there on the page. 
****Note not all units have new updates available as far as I can tell right now, there are instructions to figure out which version you are running now****