Build 10240 was a leaked build with project Astoria. If you want the build you can download it but its pretty old.
When you first get to the home screen immediately turn on airplane mode. After that go into settings and check for an update. It should say no updates found. Now you can turn airplane mode off and download app updates. The reason you must do this is because the phone will try downloading a system update but never actually starts downloading. The pending update causes all the app updates to wait until the system update is done. But it wont ever be done.
Lumia 930 - https://mega.nz/#F!JwpyBSxA!HJSIJehG6RD8YMmOd8NOGw
Lumia 640- https://mega.nz/#F!JwpyBSxA!HJSIJehG6RD8YMmOd8NOGw
Lumia 640Xl- https://mega.nz/#F!JwpyBSxA!HJSIJehG6RD8YMmOd8NOGw
Windows Phone Image Designer for flashing ROM https://mega.nz/#!k5I0zZLB!cIt5bD_qY...k1t_skydyvmnvM
Before flashing make sure you have a backup ffu. Flashing this ffu will change your product type so to revert back to normal you need to have a retail ffu file for your phone. If youve used the recovery tool before you should have a retail ffu file in C:>ProgramData>Microsoft>Packages.
DISCLAIMER: I AM NOT RESPONSIBLE FOR ANY DAMAGE DONE TO YOUR PHONE.
WARNING: SOME PEOPLE HAVE BEEN GETTING A SECOND SIM ICON ON THEIR PHONES AFTER FLASHING THESE ROMS EVEN AFTER REVERTING BACK TO WINDOWS PHONE 8.1 USING THE WPRT OR A BACKUP FFU. THE ICON WILL NOT GO AWAY AND CURRENTLY THERE IS NOT A SOLUTION TO FIX IT. IT ONLY OCCURS ON SOME PHONES NOT ALL. YOU DO THIS AT YOUR OWN RISK
1. Download the ROM for your phone from the link above.
2.Download windows phone image designer from the link above.
3.Put your phone into UEFI mode by turning it off and then booting it while holding the VOLUME UP BUTTON. You should see a lightning bolt on your screen.
4.Plug your phone into your pc through Usb and open windows phone image designer. Select the x86 folder or the x64 folder depending on your version of windows.
5.Click on the Start.bat file and windows phone image designer will launch.
6.Once booted select "Flash a Windows Phone image onto your phone" and click next.
7. Your device should show up under flashable devices. Click on change and select the ROM you downloaded.
8. Hit "Flash" and wait for the program to finish flashing.
9. Once done your phone will reboot and you will have installed build 10240 on your device! ￼