If you are trying to "download" or "install" it, you are likely either already using it or need to repair it rather than download a new file. How to Verify Your Version
no standalone download for Windows Installer 5.0 on Windows 10 64-bit . This version is already built into the operating system Windows Installer 5.0 Download For Windows 10 64-bit
If a software program prompts you to "install Windows Installer 5.0," it is usually a sign of a version mismatch corrupt system file If you are trying to "download" or "install"
This fixes corrupted registry entries for the installer. Windows Installer 5.0 Download For Windows 10 64-bit