I've noticed a few users have not been able to update directly from 1803 to 1903. But if they updated to 1809 first then 1903 was no problem.
You can download the 1809 image ISO from TechNet https://tb.rg-adguard.net/public.php they are from Microsoft servers.
If you select that method then download the latest 1809 image and once saved to your laptop then open File Explorer and double click the ISO to mount it. Go to the new drive that opens and double click the setup file. This will carry out on inplace upgrade. Check to be sure it offers to keep personal files, programs and apps.
Screenshot for download: