Failure to install Windows 10 update to 1709
First step try resting Windows Update components.
How do I reset Windows Update components?
Then carry out a clean up in PowerShell (make sure you run as admin)
DISM.exe /Online /Cleanup-Image /RestoreHealth /Source:C:\RepairSource\Windows /LimitAccess
Then run Windows update again.
If the above fails you can try the Windows Update Assistant