If you updated to Windows 10 from Windows 7, then there is a small chance the Get Back button is available in Settings > Updates and Security. For 1 month only, you can go back to the previous version of Windows without doing anything else.
Otherwise, a USB with Windows 7 or a DVD with the installation files is required. As far as I know, Windows 7 is not sold anymore, which means that you will have to get the ISO from Google. Access the search engine and simply type "download Windows 7 Ultimate ISO".
Follow this guide to transfer the ISO to a USB drive or simply burn it to a DVD using your favorite burning software.
