how to replace part of string in mongodb code example

Example 1: mongodb replace string

db.mycollection.find({}).forEach((doc, index) => {
  doc.myStr = doc.myStr.replace('foo', 'bar');
  db.mycollection.save(doc);
});

Example 2: mongodb replace document

db.collection.replaceOne(
   <filter>,
   <replacement>,
   {
     upsert: <boolean>,
     writeConcern: <document>,
     collation: <document>,
     hint: <document|string>                   // Available starting in 4.2.1
   }
)