in firebase firestore how can I update a document without overwriting subcollections? – Flutter

Just set your documents fields directly:

    FirebaseFirestore.instance
          .collection(COLLECTION_NAME)
          .doc(DOCUMENT_ID)
          .set({ "FIELD_NAME": "FIELD_VALUE" });

This will not affect the document collections.

If you want to just update some fields without affect other fields, you can use update method:

 FirebaseFirestore.instance
          .collection(COLLECTION_NAME)
          .doc(DOCUMENT_ID)
          .update({ "FIELD_NAME": "FIELD_VALUE" });

For further information:

https://firebase.flutter.dev/docs/firestore/usage/