New
#530
You have to wait until the Windows Update has finished the build upgrade download and prepared the upgrade, until you can see the Install now button in Settings > Update & Security > Update.
Clicking Install now will start unpacking and decrypting the upgrade files to C:\$WINDOWS.~BT folder. First at this point will the previous build upgrade files be replaced with the new build.
Wait until the decrypting / unpacking is over, at which point the Install now button will be changed to Restart now button:
Now you have the most recent upgrade files, including install.esd in C:\$WINDOWS.~BT folder. Don't click the Restart now button to start the upgrade before copying the install.esd file to a safe location!