Only do the following if your PC is functioning correctly and the 78% is in the Settings > Windows Update area. If it's at the blue screen, stop shutting it down. You'll damage your OS.
You can do the following manually but it's easier to have Windows fix it's own fuck up.
https://support.microsoft.com/en-us/help...bleshooter
Download the appropriate one and run it. The first menu you see will have "Advanced" link, click it and make sure to run as Administrator.
Do it once, restart. Attempt the update again, if it still has the same symptom, run the troubleshooter once more.
If it still fails, let us know. Might have to hop into RE and revert pending actions.
Once this is resolved, hop into Command Prompt (CMD) as Admin and run the following:
Code
sfc /scannow && dism /online /cleanup-image /restorehealth