I just had the same problem, only issue was I'd deleted the previous Win10 installation and didn't have the ability to write a cd/dvd as I primarily use Mac. The solution is easier than you may think provided you have the drivers (preferably have all drivers in one main folder in root of drive) and the flash that you used to install Windows 7 from.
1. Insert both flash drives
2. Boot from the Win 7 Flash
3. Choose "Repair my computer" (something like that)
4. Find the option to open the command line
5. Find your windows drive by switching drives "C:" then looking at the drive contents "dir" (it's probably not c

6. Find your flash with the drivers (if it's not on the Windows installation drive) using the steps above
7. Use the xcopy command to copy the driver folder from the flash onto the drive that you've installed Windows onto. I used "xcopy h:\swsetup f:\swsetup /h/i/c/k/e/r/y
8. Restart PC and boot into Windows
9. Use File Explorer to find the driver folder and run the applications / use Device manager and update drivers
Hope this helps (if you're still stuck)