Under Linux:
This code, named replace_applet.sh, adds a archive parameter (classes.jar) to all applet tags for *.htm files in a directory.

ls *.htm | while read x 
do
 cat $x | sed -e '
/<APPLET/ {
 s/<APPLET /<APPLET archive="classes.jar"/
}' > $x.$$
 cp $x.$$ $x
 rm $x.$$
done