The first parameter after the find is the query, in this case {} indicates an empty query (do the following statements on all rows [documents] in the collection.

<pre>
db.yourCollectionName.find( {}, { 'desiredFieldname': 1 } ).forEach(function(doc) {
  db.yourCollectionNameupdate(
      { _id: doc._id},
      { $set : { 'desiredFieldname' : doc.desiredFieldname.toLowerCase() } },
      { multi: true }
   )
});
</pre>