If I understand you correctly, you have a computer that was purchased with Windows 7 Home Premium x64 pre-installed. You installed Windows 10 on this computer and now you want to re-install Windows 7.
Assuming that the product key for Windows 7 Home Premium x64 is still legible on the COA sticker on the machine, you can reinstall Windows 7 by following the directions here: https://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install-wininstalls/clean-reinstall-windows-7/0ae269fd-167e-423b-94ef-26133f89e10c
Skip the parts that don't apply to you.
As the article linked above explains, you can get the required ISO file by using the tool downloaded from here: https://www.heidoc.net/joomla/technology-science/microsoft/67-microsoft-windows-and-office-iso-download-tool
The problem is that Microsoft is making it more and more difficult to access the latest version of the Windows 7 ISO files. Heidoc, linked above, which is the only known place to obtain a legal and untampered-with copy of these ISO files for Windows 7, is having difficulties for this reason.
As noted in the FAQ below, the August 2018 version of Windows 7 does seem to be generally available using the Heidoc tools. If you use this, you'll just have to add more updates. I don't know to what degree, if any, the Dell version has been customized.
Here is the latest information from Heidoc:
Q: I'm getting the message "This file is currently unavailable. Please check again later."
A: The file can't currently be made available. The number of different downloads for Windows 7 and Office 2010 that we can offer at the same time is very limited. The files to be unlocked next are determined by community demand. Currently available downloads are highlighted in green. You can wait until your file becomes available again, or alternatively download the Windows 7 August 2018 update, or one of the DELL OEM images.
Download options for Windows 7 and Office 2010 are currently severely restricted. New links can be generated every Wednesday from 00:00 UTC on a 'first come, first served' basis, and remain accessible until they expire.
