How to change argument of alredy running process

You can do that, if you handle that text file was changed. In the changed event of text file, you can restart that process to get new arguments in text file.

Ex:

private void Form1_Load(object sender, EventArgs e)
{
    FileSystemWatcher file = new FileSystemWatcher("C:\\Text.txt");
    file.Changed += new FileSystemEventHandler(file_Changed);
}

void file_Changed(object sender, FileSystemEventArgs e)
{
    //Restart your process here (stop and start)
}

Note: Don't always open this text file when reading arguments to execute process, because this time will lock text file, so you cannot edit text file.
Hope this help

In case of baretail there is no ipc, and if you start it, you get a new instance. You could however try to automate it, by emulating user action: sending windows messages to the process. You can use AutoIT to capture events and get an overview, or something like WinSPY[^] and Window Detective[^] to see what messages are sent.

But it would be probably easier to make a replacement of that application directly in c# that is capable of doing what you want.