Firebase create or update in Cloud Firestore

Set is equal to creating a new Document:

// Add a new document in collection "cities"
db.collection("cities").doc("LA").set({
    name: "Los Angeles",
    state: "CA",
    country: "USA"
})

Will create you a new Document named LA with the properties below. Or if you dont want to specify a ID and want Firestore to set a Auto Unique ID for you:

db.collection("cities").add({
    name: "Tokyo",
    country: "Japan"
})

Update has its own function in Firestore:

// To update age and favorite color:
db.collection("users").doc("frank").update({
    "age": 13,
    "favorites.color": "Red"
})

More examples and explanation

If you are simply using the set() function, it means that if the document does not exist, it will be created. This means that you'll be billed for a write operation. If the document does exist, its contents will be overwritten with the newly provided data, unless you specify that the data should be merged into the existing document, as follows:

var setWithMerge = yourDocRef.set({
  yourProperty: "NewValue"
}, { merge: true });

This will also represent a write operation and you'll also be billed accordingly. If you are want to update a property as in the following code:

return yourDocRef.update({
    yourProperty: "NewValue"
})
.then(function() {
    console.log("Document successfully updated!");
})
.catch(function(error) {
    console.error("Error updating document: ", error);
});

It also means that you are performing a write operation. According to the official documentation regarding Firestore usage and limits, there is no difference between a write operation and a update operation, both are considered write operations.