I hoped to find a documentation of the bug fixes and improvements in the update to version X7.3 (1 April 2015) on page <http://endnote.com/downloads/available-updates>, but there was none. Actually, I was also hoping to see a reference to this update on the home page, which would be a most logical spot.
The Release Notes for EndNote X7.3 can be found in the ReadMe file included with the installation of the software. You can find this in the Program Files or Program Files (x86) > EndNote X7 folder on Windows or the Applications > EndNote X7 folder for Mac. This will include general information about what is included in the current release.