The folks over at WordPress have released the first minor revision for WordPress 3.0 in the form of WordPress 3.01. No new features or anything like that but it does fix about 50 issues according to this entry at the WordPress Codex.
If you really want to delve into the individual fixes you can check out the WordPress trac for release 3.01.
You can update to 3.01 by visiting your Admin backend and clicking on the Dashboard > Updates menu item.
Just follow the instructions from there.
Of course you can always just download WordPress 3.01 in its entirety and manually upload the files to your host. If you can do the automatic update I recommend that as it is the best method in my opinion.