If the actual msiexec.exe file or its dependent system libraries are corrupted, Windows can automatically replace them with fresh copies from its internal system image. Open as an administrator.

If you absolutely need to reinstall or repair the Windows Installer engine on Windows 10 64-bit, you must download the latest Cumulative Update for your specific version of Windows 10.

Windows Installer receives its security patches and functional updates through the standard Windows Update service.

Thus, the "new" Windows Installer 5.0 for Windows 10 64-bit is always the one shipped with your current build.

Since you cannot download a separate file, you must use official system tools to ensure your installer is current and functioning correctly. Option A: Use Windows Update

In 99% of cases, You cannot install an older version of Windows Installer 5.0 over the newer version already in Windows 10. If you try to download a standalone "Windows Installer 5.0" package from 2009, Windows 10 will block the installation because your OS already has a newer servicing stack.