Thank you for your suggestions. In my multiple tries to get going with the update from version 1709 to 1803 in the past , I do recall, but not exactly, having gone along the route outlined in the first part of your suggestion, without success. I even downloaded version 1803 from a Windows website, had it successfully installed and used for just over a week where-after Windows Update {WU) overwrote it and sent me back to version 1709. Thereafter when I attempted to install it for the 2nd time from my USB drive, WU returned a message that it was an incorrect version.
WU,, it seems, wants me to stick to version 1709, because it has been providing updates for version 1709 right up to the present day. For example, just about half an hour ago, the following appeared on my WU window:
"• 2019-02 Cumulative Update for Windows 10 Version 1709 for x86-based Systems (KB4486996)
Status: Awaiting restart."
Although the update is dated for February 2019, it was downloaded today when I opened Windows Update. You will observe that they are providing updates to version 1709 right up to at least February this year. This seems to indicate that if one has version 1709, one should stick to it seeing that 1709 is being kept up to date.
Having expended a considerable amount of time and effort, without any joy, I have decided to let Windows Update do its thing.
Your effort to assist is much appreciated. Many thanks.