If you have FTP access to the file then yes, download it via FTP, edit it, and re-upload it. Usually it's faster to just SSH into the server and edit it with vim or nano though.
You can reduce the refresh rate by increasing the 60000 in the line I quoted earlier. You can't increase the rate though -- Centova Cast only polls the server once per minute so having the javascript refresh more frequently than that won't yield any benefit.
In v3.0 you'll be able to change the poll frequency (although we don't really recommend this to avoid hammering the server).