If you're not using Debian, you'll have to modify your PATH environment variable, or create a link to the program you want in a place with a higher precedence. $PATH is search sequentially, and the first match is the one used, so if you 'echo $PATH', you'll probably find that /usr/kerberos/bin is ahead of /usr/bin. $PATH is usually defined globally in /etc/profile, so you can look there and rearrange it if you like.
Otherwise, you'll probably find that /bin or some other directory (perhaps /usr/local/bin) comes before even /usr/kerberos/bin, and you could create a link to your preferred ftp binary there.
Finally, it's possible that that is the only ftp client installed on your machine -- grab netkit and install a basic copy.
If you are bychance using Debian, 'man update-alternatives' should help you out.