Microsoft's changelog for this release for the Threshold branch (build 10586.420) is:
This update includes quality improvements and security fixes. No new operating system features are being introduced in this update. Key changes include:
- Improved reliability of Internet Explorer 11, Microsoft Edge, Cortana, audio playback, audio playback in Groove Music app, Maps app, Miracast, and Windows Explorer.
- Fixed issue with balloon tip notifications always appearing in the upper left side of the screen.
- Fixed issue that caused VPN to not work properly when switching between different network interfaces (for example, switching between Wi-Fi and cellular).
- Improved Narrator’s ability to read bulleted lists, hyperlinks, and image information.
- Fixed issue in location that caused navigation apps to lag behind user’s actual location.
- Improved performance of loading webpages in Internet Explorer 11 when roaming user profiles are used.
- Fixed issue that caused phones to stop ringing from an incoming call, if interrupted by an SMS.
- Fixed issue that caused some phones to be unable to re-add a primary Microsoft account without resetting after upgrading from Windows Phone 8.1.
- Fixed additional issues with Microsoft Edge, Internet Explorer 11, Bluetooth, Cortana, Wi-Fi, Windows Camera app, revised daylight saving time, USB, TPM, Graphics, Group Policy, downloading music or movies purchased through the Windows Store, Network Diagnostics and Windows Explorer.
- Fixed security issues with Internet Explorer 11, Microsoft Edge, Server Message Block (SMB) Server, Microsoft Graphics Component, Group Policy, DNS Server, Windows Diagnostic Hub, Kernel mode drivers, Microsoft Windows PDF, Windows Structured Query, Adobe Flash Player, JScript and VBScript, and Web Proxy Autodiscovery Protocol (WPAD).
Other changes include:
- Improvements to Cortana, including a fix for a problem that sometimes resulted in a crash when performing a search, and corrections to inaccurate times after Daylight Savings Time (DST) on meeting cards and reminders.
- Fix for a problem that could result in unexpected battery drain while syncing mail using IMAP.
- Improvements to the Maps app, including better stability, improved accuracy, and reduced latency with driving directions.
- Improved reliability of syncing text messages from the phone to the cloud.
- Reduced amount of time it takes some phones to completely power off after “Goodbye” disappears.
- Fix for a problem that caused some phones unable to install keyboard and languages without resetting after upgrading from Windows Phone 8.1.
- Reduced the number of days a phone can go without restarting to receive an update from 14 days to 7 days.
- Fix for a problem that caused Project My Screen to stop working on some phones.
We still have a good month or two before Microsoft starts to think about pushing the 'Redstone' (14xxx) branch to production devices, right now we're still largely in a 'Threshold' world. You can update, as usual, by going into Settings/Update & Security/Phone update and 'Check for updates'. Make sure your battery is above 40% and/or plug the phone in!
In addition to older Windows Phones upgraded to Windows 10 Mobile, by necessity, through the Insiders programme (and time's running out, by the way), this update may also be of interest to all those who upgraded officially or who have a Windows 10-native phone (e.g. Lumia 950) and who want the latest bug fixes but who don't feel ready for the full 'alpha' nature of the Redstone builds!
I'm updating a test Lumia 920 and 1020 (I know, I know) as I speak, your comments welcome if you have anything to add for this specific build.