There is a registry fix to enable 10240 to upgrade from Windows update for early builds such as 10074.
copy the code to notepad, select save as, save as type: All files, and save as TH1.reg or use the attachment provided.
Windows Registry Editor Version 5.00
open the file and follow the instructions to merge the registration details into the registry.
Then go to settings and updates, check for updates and
TH1 Professional 10240 release preview should start to download and install.
I have just done this and after an uneventful upgrade, the 10240 Professional was running fine and activated. The 10074 install that was upgraded had not been activated, and had the install default product key of NKJFK-GPHP7-G8C3J-P6JXR-HQRJR.
Otherwise upgrade using the Iso of the RTM version using Media Creator Tool: Windows 10 ISO Download - Windows 10 Forums