Windows 10 Slow/Fast Ring Upgrade

Try downloading the ISO from the insider page, option 4 in Tut below

Download Windows 10 ISO File

I didnt see this part below on our W10 tuts, but its probably there, here from windows central

How to install the May 2020 Update using ISO preview file
Alternatively, this time around, Microsoft is also making available an ISO preview file of the final release of Windows 10, which any participant of the Windows Insider Program can download to perform an in-place upgrade or clean installation.

To install the May 2020 Update early using an ISO file, use these steps:

Open Windows Insider Preview Downloads page.
Sign in with your Windows Insider Program credentials.
Under the "Select edition" section, select the Windows Insider Preview (FAST) – Build 19041 option.

Download Windows 10 version 2004 ISO preview


Click the Confirm button.
Select the language that matches your location.
Click the Confirm button.
Click the 64-bit Download button.

Windows 10 ISO 64-bit download (preview)


Quick tip: Click the 32-bit Download button, if this is the architecture already installed on your computer. You can find out whether your device is running 64-bit or 32-bit on Settings > System > About.

Save the file to your computer.
Open the folder containing the Windows 10 May 2020 Update ISO file.
Right-click the ISO file and select the Mount option to mount and open the ISO file with the installation files.

Windows 10 mount ISO file


Double-click the Setup.exe file.

Windows 10 Setup file


Click the Change how Windows Setup downloads updates option.

Windows 10 Setup download updates option


Check the Not right now option.

Change update options on Windows 10 Setup


Click the Next button.
Click the Accept button to agree to the Microsoft terms.
Confirm that the Keep personal files and apps option is checked.

Windows 10 May 2020 Update ISO upgrade


Quick tip: If the option isn't selected, click the Change what to keep option and select the preferred option.

Click the Install button.
Once you complete the steps, the setup will take over and upgrade your device to Windows 10 May 2020 Update using the latest available preview build.

After the upgrade, remember to go to Settings > Update & Security > Windows Update, and click the Check for Updates button to download the latest update and bump the version number to build 19041.207.

How to get the Windows 10 May 2020 final release before anyone else | Windows Central

It worked for me