You'll need to use the Media Creation Tool to create a Windows 10 installation ISO file. Make sure you have a license to install Windows 10, then follow these steps:
From the Windows 10 download page, download the Media Creation Tool by selecting Download tool now, and then run the tool.
In the tool, select Create installation media (USB flash drive, DVD, or ISO) for another computer > Next.
Select the language, architecture, and version of the Windows operating system you need and select Next.
Select ISO File > Next, and the tool will create an ISO file for you.
To use the ISO file as a backup to your computer or another computer, burn it to a DVD.