HandBrake

HandBrake is a open-source tool, built by volunteers, for converting video from nearly any format to a selection of modern, widely supported codecs.

By clicking on the prompts that appear using the HandBrake installer, I automatically downloaded windowsdesktop-runtime-6.0.12-win-x64.exe.

After installing that, without needing to restart, I was able successfully to use HandBrake to convert a .webm file to an .mp4 file.