The fourth major milestone for Windows 10, Redstone 2, has been delayed until early 2017. The release is slated to coincide with hardware releases from Microsoft. Redstone 1 is still slated for June.
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Previous reports indicated that Microsoft would deliver its Windows 10 Redstone updates in two waves this year - but while RS1 is said to be on track for June, RS2 has reportedly been 'pushed back'.