Here's a general, somewhat robust solution to this (copied from my own comment on Steam forums):
Works only on Windows (or Wine/Proton for Linux users).
- Go to the game's installation folder,
- Look for a file named installscript.vdf, open it using a text editor (like Notepad)
- Look for a line that says "HasRunKey". To the right of that, there should be a path string which looks something like
- Open RegEdit.
- Go to that path (if you can't find it, you might want to look at Wow6432Node instead)
- Once you find the registry key that you need, delete it.
- Start the game through Steam again, the install script should run.
Note that this only works for some game-specific scripts, but not all of them. I used this method to re-install Rockstar Games Launcher for GTA IV, but it doesn't work for things like C++ Redistributables. I guess those are handled separately.
Alternatively, you could just look at the actual script to see what it does, and just do it manually. Using GTA IV as an example again, you may find this line in its install script:
"Process 1" "%INSTALLDIR%\\GTAIV\\Redistributables\\Social-Club-Setup.exe"
Just means that you need to find the install folder, then go to
GTAIV\Redistributables\ and run