It's sealed with a rubber oil seal, which you can replace pretty easily if you're careful. First you'll need to either drain the oil or tip the quad up on the side, put a jack or whatever else you have underneath the handlebar end so it doesn't completely flip over. Remove the shifter/shift linkage from the shaft. Then, being very careful, you should be able to pick the seal out of the case and remove it. I usually use the corner of a razor blade to get under the edge a little and work my way around it, prying it up a little bit at a time until you can pull it off, over the shaft. You can then slide your new seal back over the shaft and work it into place. Put a little oil on the seal before you try installing it, it'll slide right in easier. Don't force it or use any kind of sharp tools, just push it in by hand or use something smooth and non-metallic to work it in, a popsicle stick would probably work well. Then you're all set, just put it back together.
If you need to find the part online, in most parts diagrams it will be listed under "shift cam fork", on ronnie's mail order it's part #10 on the diagram.