set_wm_class.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>

int main(int argc, char *argv[])
{
	unsigned long value;
	char *terminatedAt;
	XClassHint class;
	Status status;
	Display *display;
	Window window;
	
	if ( argc != 3 ) {
		printf( "Usage: %s <window id> <window class>\n", argv[0] );
		return 1;
	}
	window = strtoul( argv[1], &terminatedAt, 0 );
	if ( *terminatedAt != '\0' ) {
		printf( "Could not parse window id: %s\n", argv[1] );
		return 2;
	}
	
	display = XOpenDisplay( NULL );
	status = XGetClassHint( display, window, &class );
	if ( !status ) return 4;
	
	XFree( class.res_class );
	class.res_class = strdup( argv[2] );
	printf("Setting WM_CLASS of window %lu to \"%s\", \"%s\"\n", window, class.res_name, class.res_class );
	XSetClassHint( display, window, &class );
	
	XCloseDisplay( display );
	XFree( class.res_name );
	XFree( class.res_class );
	return 0;
}

You could just use xdotool..

apt install -y xdotool
xdotool search --name "Title of App" set_window --class "New WM Class"

Thanks @Fmstrat! Copied that to the StackOverflow question that prompted this gist.