How to restart fvwm2 from the command line?

What you want is either to make fvwm re-read the configuration or restart it completely. Additionally, I'm not totally sure that -c does what you want.

The fast or dirty way would be:

killall fvwm
fvwm

Note: Please do not execute this on a BSD or Solaris system, as killall will kill the init process there. killall processname is Linux only.

But I'm not sure if this wouldn't destroy some window information (location, dimensions etc.). Or, you could try replacing the current session:

fvwm --replace

That should end the old fvwm process, launch a new one and even keep the windows correctly.

If fvwm is your x-session, you could still try to use FvwmCommand, the external command interface which seems to be able to send commands to the currently running fvwm.