Apple released the first beta of OS X 10.10.5 to public beta testers on Thursday. The public beta release comes just a few days after the same beta was seeded to developers for testing.
The new beta, build 14F6a, (the same build seeded to developers earlier), can be downloaded via the Update tab in the Mac App Store. Your Mac will need to be enrolled in the Apple Software Beta Program for the update to appear.
MacRumors notes 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.
The OS X 10.10.5 beta contains stability and compatibility fixes, as well as security improvements.