Flutter update a field in firebase with query from subcollection?

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Flutter

I have been able to query a data in a subcollecion directly using collectiongroup with the following code in flutter:

What I have tried:

final uid = FirebaseAuth.instance.currentUser!.uid;
QueryDocumentSnapshot<map<string, dynamic="">>? selectedRide;

StreamBuilder(
stream: FirebaseFirestore.instance
.collectionGroup('Requests')
.snapshots(),
builder: (BuildContext context,
AsyncSnapshot<querysnapshot<map<string, dynamic="">>> snapshot)
{ final request = snapshot.data!;

return ListView.builder(
itemCount: snapshot.data!.docs.length,
itemBuilder: (context, index) return Card(
onTap: () {} ////update data onTap of this list view.
//Let's say the data is : request.docs[index].get('Full Name')
How do I update it?
)})
)