I have a firestore collections named users, each users have a generated id with a field score :
users 0e8X3VFL56rHBxxgkYOW score : 4 3SeDjr
You can get all the documents in the collection, get their id's and perform updates using those id's:
db.collection("cities").get().then(function(querySnapshot) { querySnapshot.forEach(function(doc) { doc.ref.update({ capital: true }); }); });