Click on the "Download Now" button. This will give you the online installer ( Razer_Cortex_Installer.exe ). Do not run it yet.
Go to the official Razer Cortex website: https://www.razer.com/cortex razer cortex offline installer
Instead of running the file, use a download manager or simply wait. Some advanced users use browser developer tools (F12) to capture the direct download URL. However, the easiest method is to start the online installer and then locate the temporary download location. Click on the "Download Now" button