I just updated to VS2017 15.3.1 preview 1.
In Extensions and Updates, I show Updates (5) and they are for Xamarin Preview (5)
I clicked on each one and each update button. They download and now there's a tiny clock icon in the upper right of each of the 5 items. I can see at the bottom : "Your changes will be scheduled. the modifications will begin with all Microsoft Visual Studio windws are closed".
I can see each item listed in 'Scheduled for Update' in the bottom right of the window.
However, I've closed VS2017 - and waited a while. When I open it back up again, Extensions and Updates shows the same 5 Xamarin Preview updates available. The tiny clock icon is gone. If I click on a row, there's an update button again which will download the same update Again.
Unsure how to proceed? Back in the old days, updates would get stored in C:\Users\MyName\AppData\Local\MonoDevelop-4.0\Cache\TempDownload and I could run them myself, but ... that folder is empty.