Shell Script modification

Hi,

I have below shell script to get files from ftp server.
I need to change this to SFTP server. Can some one guide how to change this script to point to SFTP server.

HOST='some.site.com'
USER='yourid'
PASSWD='yourpw'
FILE='file.txt'

ftp $HOST <<END_SCRIPT
user $USER
$PASSWD
put $FILE
quit
END_SCRIPT
exit 0