You need to download the Windows Media Creation Tool from Microsoft which will then assist you with downloading the Windows 10 ISO and transferring it to your USB key.
Windows Media Creation Tool
- Click Create installation media (USB flash drive, DVD, or ISO file) for another PC, then click Next
- You can uncheck the Use the recommended options for this PC if you want to choose other custom options like choosing to grab an installer to be able to install both 32 & 64 bit versions, then click Next
- Select either USB flash drive or ISO file, then click Next
- Choose the USB flash drive you want to write to and click Next
- You will see a percentage progress and Finally it will say Your USB flash drive is ready, click Finish
You now have a flash drive you can use to upgrade or do new Windows 10 installs.