If you want to install or reinstall Windows 8.1 and don’t have installation media, you can get the step by step from Windows support page. You’ll need to use a PC with a reliable Internet connection and either save the installation files to a USB flash drive or burn an ISO file to a DVD.
You will need to create the media disk and then save it or burn the ISO to a flash drive or DVD.
Oh, and it’s FREE! (Of course. you will need a valid Windows 8.1 license key to use your ISO)