Usually you can lean on webcit, the citadel commandline client or setup to modify your citservers config.
If all of them fail, here is the cumbersome way, which otoh only requires sendcommand, an editor of your choice (it should show you the line number for convenience) and a good eye.
we will obtain the current configuration from citserver using:
sendcommand conf get > /tmp/cfg.txt
now we can use the documentation to know which line is which setting, change it, save it, and send it back to citserver using:
sendcommand conf set < /tmp/cfg.txt
Now restart citserver to make the changes taking effect.