I am currently working with Drupal 6. I just want to know how to convert all the underscores in the URL of my website to hyphens.
At admin/build/path/pathauto change the separator from underscore to hyphen and make sure the other punctuation settings are the way you like (the ones that say "Replace by separator").
Then at admin/build/path/delete_bulk delete all the aliases that you want to change.
Then recreate them at admin/build/path/pathauto by checking "Bulk generate aliases for nodes that are not aliased" in Node paths, User paths etc. as needed.