Windows 10 Build 10159 is now available for fast ring insiders via Windows Update! You can update right now via Windows Update, and can expect 300 new fixes with this build. Plus, Gabe Aul tweeted on Twitter that there's some kind of surprise in this build, which is interesting to say the least.
Build 10159 is just one build newer than 10158, which was released only yesterday, so what this means is there probably isn't any dramatic changes in. Of course that's just an assuption, so until we get our hands on the build we're going to have to wait and see what this 'surprise' is.
Check for updates now via Windows Update and you should hopefully see it! If not, give it some time as the update is still rolling out globally.