$ echo "hahahaa_the_cat_hahahaa" | sed "s/_///g"

$ echo "hahahaa_the_cat_hahahaa" | sed 's|_|/|g'

$ echo foo | sed 's|f|g|'
goo
$ echo foo | sed 'safaga'
goo

$ echo "hahahaa_the_cat_hahahaa" | sed 's|_|/|g'
hahahaa/the/cat/hahahaa

$ echo "hahahaa_the_cat_hahahaa" | sed 's/_///g'
hahahaa/the/cat/hahahaa