If you go to start - settings - recovery. Under Reset this PC select remove everything it will reset Windows 10 as a clean install and coulds take several hours to complete.
If you download the Media creation tool from CLICK HERE you can download and create Windows 10 DVD or USB to reinstall windows 10 as an upgrade or as a clean install. As you have upgraded from Windows 8.1 to Windows 10 you will have a digital license that will reactivate as soon as you are connected to the internet.
To upgrade use the factory reset option to reinstall windows 8.1 and then upgrade from withing windows to Windows 10.
To do a clean install you will need to boot from the DVD or USB you have created. Note you may need to check and download some drivers.