

The new beta can be downloaded from the Software Update mechanism through the Mac App Store on Macs enrolled in the Apple Beta Software Program. The pre-release software's build number is 14F6a, the same as the OS X 10.10.5 beta seeded to developers earlier this week.
It also appears that the first OS X 10.10.5 beta is now showing up in Software Update for developers who were unable to see the pre-release software following its release on Tuesday.
OS X 10.10.5 beta includes stability, compatibility and security improvements.
OS X 10.10.4, the current version of OS X Yosemite, was publicly released on June 30 with multiple bug fixes and under-the-hood performance improvements, including the removal of the problematic Discoveryd process.
Article Link: Apple Seeds First OS X 10.10.5 Beta to Public Beta Testers