Hello,
Please refer the guided walk-through mentioned in this support article to fix update issues:
https://support.microsoft.com/en-us/help/10164/fix-windows-update-errors
If that doesn't helps you, try this:
Open administrative Command Prompt and type following commands one-by-one followed by Enter key.
net stop wuauserv
net stop cryptSvc
net stop bits
net stop msiserver
Ren C:\Windows\SoftwareDistribution SoftwareDistribution.old
Ren C:\Windows\System32\catroot2 Catroot2.old
net start wuauserv
net start cryptSvc
net start bits
net start msiserver
Close Command Prompt and see if you can proceed to install V1903.
If issue still persists, try to reset Windows Update components:
https://docs.microsoft.com/en-us/windows/deployment/update/windows-update-resources
If nothing works, you can upgrade to Windows 10 V1903 using ISO file.
Refer this step-by-step guide:
https://www.kapilarya.com/how-to-upgrade-to-windows-10-using-iso-file
Let us know if this helps!
Note: Included link in this reply refers to blog post by a trusted Microsoft MVP.