Hi Ben,
I am Sumit, an Independent Advisor and a 2-Year Windows Insider MVP here to help.
Try updating to 1903 using Media Creation tool and if it fails let us know.
The fastest way to get the upgrade is through Media creation tool or ISO file.
Please download Media Creation Tool from the link below and select Upgrade this PC at the first screen.
https://go.microsoft.com/fwlink/?LinkId=691209All steps, including upgrading using ISO file and Media creation tool, are covered here:
https://www.yourwindowsguide.com/2019/05/update...Content on the above blog is written by a Microsoft MVP so it is safe.