One of my VMs is at 10074 and seems not to want to update. Last week I looked into this and saw that I needed to log in using my Microsoft login. I did that and it seems not to matter. My other VM upgraded to 10240 before July 29th.
I tried the fix I found at [Forum FAQ]How to upgrade to Windows 10 build 10240 if Windows Update is unavailable and it did not work. When I went into Advanced Options after changing the registry entry and trying to update it had an entry at the bottom of the Update & Security > Windows Update > Advanced Options that says "The selected branch is not set to receive preview builds" and gives me the option to switch back to fbl_impressive. (I still get the Definitions Updates for Windows Defender.) winver tells me this update expires at 10/1/2015.
Did I wait too long and miss the boat?