As Rolf says, there's a ton of info about this on the site. It's been discussed many times. It depends on whether you're prepared to do it yourself, which is a quite a bit of work but not by any means impossible, (
http://www.sl113.org/forums/index.php?topic=18228.50) or whether you want someone else to do it for you.
In either case, you'll need to confirm what diff you have now, and then figure out what you want to change it to. Again, all the info is here on the site. (Your diff ratio is stamped on a machined surface at the base of the diff on the rear casing, and is on the data card if you have a copy).
Note that there are pros and cons of changing the diff. Again, I'd suggest a few hours trawling through the myriad posts on the site.