How to Download Older Windows 10 ISOs
When the latest Windows 10 feature update is released to the public, Microsoft makes the previous version unavailable. There are times, however, when the current version isn’t what we are looking for.
Fortunately, there is a bat file, which allows you to choose to install any of the older versions of Windows 10 and download it directly from Microsoft's servers.
If you do need to download older Windows 10 versions, below how to quickly do that.
Everything is in a text file, which means you can see for yourself if there is any is unusual about the script. This option is for pro users, developers, and others who might need access to older Windows 10 feature updates for specific requirements.
1. Head over to this GitHub Gist link.
2. Click on the Download ZIP button on the top-right corner.

3. Open the zip file from your “Downloads” folder and then run the MediaCreationTool.bat file as an administrator.

4. You will now see the below screen with all of the available versions of Windows 10:

5. Click on the version that you want to download. Version 2009 aka 20H2, 2004 aka 20H1, and 1909 are recommended. In the below screenshot version 2004 has been selected.

6. From here, the update process will start as normal.

7. Microsoft will ask for some permissions and legal agreements and you will be asked whether to upgrade the same device or download the ISO file for another device.
Even if you want to upgrade now, I suggest that you create an ISO so you can use it at a later time if necessary. Once the ISO is created, it can be used on any Windows physical machine or virtual machine.