You can use the following methods to rename fields in MongoDB:

Method 1: Rename One Field

db.collection.updateMany({}, {$rename:{"oldField":"newField"}}, false, true)

Method 2: Rename Multiple Fields

db.collection.updateMany({}, {$rename:{"old1":"new1", "old2":"new2"}}, false, true)

Method 3: Rename Subfield

db.collection.updateMany({}, {$rename:{"field.oldSub":"field.newSub"}}, false, true) 

Note that the false, true in the $rename function stands for {upsert:false, multi:true}.

You need the multi:true to update the field name in all of your documents.

The following examples show how to use each method with a collection teams with the following documents:

db.teams.insertOne({team: "Mavs", class: {conf:"Western", div:"A"}, points: 31})
db.teams.insertOne({team: "Spurs", class: {conf:"Western", div:"A"}, points: 22})
db.teams.insertOne({team: "Jazz", class: {conf:"Western", div:"B"}, points: 19})
db.teams.insertOne({team: "Celtics", class: {conf:"Eastern", div:"C"}, points: 26})
db.teams.insertOne({team: "Cavs", class: {conf:"Eastern", div:"D"}, points: 33})
db.teams.insertOne({team: "Nets", class: {conf:"Eastern", div:"D"}, points: 38})

Example 1: Rename One Field

We can use the following code to rename the team field to new_team:

db.teams.updateMany({}, {$rename:{"team":"new_team"}}, false, true) 

Here are what the documents look like now:

{ _id: ObjectId("62017ce6fd435937399d6b58"),
  class: { conf: 'Western', div: 'A' },
  points: 31,
  new_team: 'Mavs' }
{ _id: ObjectId("62017ce6fd435937399d6b59"),
  class: { conf: 'Western', div: 'A' },
  points: 22,
  new_team: 'Spurs' }
{ _id: ObjectId("62017ce6fd435937399d6b5a"),
  class: { conf: 'Western', div: 'B' },
  points: 19,
  new_team: 'Jazz' }
{ _id: ObjectId("62017ce6fd435937399d6b5b"),
  class: { conf: 'Eastern', div: 'C' },
  points: 26,
  new_team: 'Celtics' }
{ _id: ObjectId("62017ce6fd435937399d6b5c"),
  class: { conf: 'Eastern', div: 'D' },
  points: 33,
  new_team: 'Cavs' }
{ _id: ObjectId("62017ce6fd435937399d6b5d"),
  class: { conf: 'Eastern', div: 'D' },
  points: 38,
  new_team: 'Nets' } 

Notice that the team field has been renamed to new_team for every document.

Example 2: Rename Multiple Fields

We can use the following code to rename the team field to new_team and the points field to new_points:

db.teams.updateMany({}, {$rename:{"team":"new_team", "points":"new_points"}}, false, true) 

Here are what the documents look like now:

{ _id: ObjectId("62017ce6fd435937399d6b58"),
  class: { conf: 'Western', div: 'A' },
  new_team: 'Mavs',
  new_points: 31 }
{ _id: ObjectId("62017ce6fd435937399d6b59"),
  class: { conf: 'Western', div: 'A' },
  new_team: 'Spurs',
  new_points: 22 }
{ _id: ObjectId("62017ce6fd435937399d6b5a"),
  class: { conf: 'Western', div: 'B' },
  new_team: 'Jazz',
  new_points: 19 }
{ _id: ObjectId("62017ce6fd435937399d6b5b"),
  class: { conf: 'Eastern', div: 'C' },
  new_team: 'Celtics',
  new_points: 26 }
{ _id: ObjectId("62017ce6fd435937399d6b5c"),
  class: { conf: 'Eastern', div: 'D' },
  new_team: 'Cavs',
  new_points: 33 }
{ _id: ObjectId("62017ce6fd435937399d6b5d"),
  class: { conf: 'Eastern', div: 'D' },
  new_team: 'Nets',
  new_points: 38 } 

Notice that the team field and the points field have both been renamed in each document.

Example 3: Rename Subfield

We can use the following code to rename the div subfield within the class field to division:

db.teams.updateMany({}, {$rename:{"class.div":"class.division"}}, false, true) 

Here are what the documents look like now:

{ _id: ObjectId("62017e21fd435937399d6b5e"),
  team: 'Mavs',
  class: { conf: 'Western', division: 'A' },
  points: 31 }
{ _id: ObjectId("62017e21fd435937399d6b5f"),
  team: 'Spurs',
  class: { conf: 'Western', division: 'A' },
  points: 22 }
{ _id: ObjectId("62017e21fd435937399d6b60"),
  team: 'Jazz',
  class: { conf: 'Western', division: 'B' },
  points: 19 }
{ _id: ObjectId("62017e21fd435937399d6b61"),
  team: 'Celtics',
  class: { conf: 'Eastern', division: 'C' },
  points: 26 }
{ _id: ObjectId("62017e21fd435937399d6b62"),
  team: 'Cavs',
  class: { conf: 'Eastern', division: 'D' },
  points: 33 }
{ _id: ObjectId("62017e21fd435937399d6b63"),
  team: 'Nets',
  class: { conf: 'Eastern', division: 'D' },
  points: 38 } 

Notice that the div subfield within the class field has been renamed to division in each document.

Note: You can find the complete documentation for the $rename function here.

Additional Resources

The following tutorials explain how to perform other common operations in MongoDB:

MongoDB: How to Add a New Field
MongoDB: How to Remove a Field
MongoDB: How to Count Distinct Values in Field