KB3085534 is a new update published by Microsoft on May 20 to correct a problem in Office 2016 and causing the synchronization process in OneNote to fail. Once the update appears, you can select Download and install now. We'll try again later. Windows users still on the version 1803 should get the build 17134.766, and there are also new cumulative updates for the versions 1709, 1703, 1607 1507. This is part of Microsoft's attempt to make Windows Update less surprising: feature updates are offered separately from regular updates, because feature updates take a long time to install and regular updates don't (or at least, shouldn't).
Microsoft notes that it may happen that Windows 10 can not be restored to an earlier version and that users receive the Stop error (0xc000021a) instead.
Microsoft is however not offering the download to all seekers at present as they are slowly throttling up this availability, while they carefully monitor data and feedback. Both of them can be downloaded from the Microsoft Download Center using the links here. However, if you don't want to wait to upgrade using the recommended option, there are multiple ways to upgrade manually as soon as the update becomes available.
For those, running older versions that are unsupported or near to end of support, they will automatically update to the newest feature update.
That rollout starts today: while previously one needed to be in the Insider Program (or have a source such as an MSDN subscription) to download and install version 1903, it's now open to everyone through Windows Update.
It has also been confirmed that if you found a workaround after the first set of borkage, the sames ones should work for this, until it can be fixed properly.
Starting in June, Microsoft will begin updating Windows version 1703 (April 2018 Update) and earlier to version 1903.