W10 1809 won't upgrade

If it is right now, you may really just wait, as far as you have an internet connection.
If the update is denied for longer than you may have a look at the windows update troubleshooter:
https://support.microsoft.com/en-us/sbs/windows/fix-windows-update-errors-18b693b5-7818-5825-8a7e-2a4a37d6d787?ui=en-US&rs=en-US&ad=US

Or it is a fireall issue. 
If you are connected to a coporate network, there may be possibly other mechanism how you get updates. 
 

Are you sure you have Internet  access?  Check with a b browser that you can reach

https://download.microsoft.com 
and
http://download.windowsupdate.com 

If you can't get there with your browser, then you have a network issue not a Windows Update issue.  

I would second Robert's recommendation but with a slightly different perspective.  When installing major Windows 10 updates (such as 20H2 or 21H1) my preference is to use his "last resort" of downloading the ISO, mounting it, and installing from there.

I had the exact same problem when one of my copies of 1909 would constantly report an error trying to update to 20H1 and 20H2 and I found that the only way to resolve the error was to fall back on the download ISO method that Robert is referring to. I've also had the same problem with previous builds where security updates would still apply just fine, but a build increase needed a non-destructive upgrade. I even wrote an article about it here.

• Non Destructive Repair of Windows 10 - Answers to commonly asked questions

Hope it helps.

Best, Andrew

Lots of good stuff....I will do the Microsoft download burn a DVD and do the upgrade from there...
Many thanks...