The updates, which bring both applications to version 9.1.4, primarily serve to add compatibility with projects created in GarageBand for iPad; in addition, a bunch of small issues with performance, stability, 64-bit, automation, audio editing, control surface support, and preferences have been addressed. If you’re inclined to know what specifically was addressed, you can read the full release notes for Logic Express and Logic Pro on Apple’s Website.
To install the updates—which come in at 138.74MB for Logic Express and 192.48MB for Logic Pro—you can fire up Software Update or head over to Apple’s Support Website. The updates require OS X 10.5.8 or later.