Using WS_FTP, right click on the file you wish to modify in the right window (those on the left are files on your local hard drive). You should get the following menu:

Select chmod (UNIX). This will bring up a "Remote file permissions" window that should look like this:

Set the permissions for your .cgi file to what is shown.

Do NOT make .cgi scripts "Group" and "Other" writable!! Doing so will allow non-privileged users to update/modify your script.