Find on Steam forum
Go to your Steam folder, Steam\steamapps\common\Elite Dangerous and start the EDlauncher.exe
Select Elite/Horizons/Odyssey bottom list > check/click UPGRADE
(may have to use arrow keys to scroll the list)
2nd option:
Open the launcher as admin from the directory and in options disable Enable Cache Check.
Then CLOSE the launcher and reopen as admin and then click "upgrade". Restart after update.
-be aware that the upgrade can be slow, so patience is in order.