I created a variation for your code and I made it a little more Generic

Action Name: updateIndexValue

you have the following variables

collectionName <String>:
The name of the collection you are updating.

listFieldName<String>:
The name of the field containing the list on the collection.

index<int>:
The index in the list you want to modify (starts at 0).

newValue<String>:
The new value you want to update to.

uid<String>:
This is a uid you have to set in your collection to find the document.

// Automatic FlutterFlow imports\nimport '/backend/backend.dart';\nimport '/backend/schema/structs/index.dart';\nimport '/flutter_flow/flutter_flow_theme.dart';\nimport '/flutter_flow/flutter_flow_util.dart';\nimport '/custom_code/actions/index.dart'; // Imports other custom actions\nimport '/flutter_flow/custom_functions.dart'; // Imports custom functions\nimport 'package:flutter/material.dart';\n// Begin custom action code\n// DO NOT REMOVE OR MODIFY THE CODE ABOVE!\n\nimport 'package:cloud_firestore/cloud_firestore.dart';\n\nFuture updateIndexValue(\n  String collectionName,\n  String listFieldName,\n  int index,\n  String newValue,\n  String? uid,\n) async {\n  // Add your function code here!\n\n  if (uid != null) {\n    // Get a reference to the Firestore collection.\n    final CollectionReference myCollection =\n        FirebaseFirestore.instance.collection(collectionName);\n\n    // Query the collection for documents with 'uid' equal to the provided uid and limit the result to 1.\n    final QuerySnapshot querySnapshot =\n        await myCollection.where('uid', isEqualTo: uid).limit(1).get();\n\n    // Get the list of documents returned by the query.\n    final List<QueryDocumentSnapshot> documents = querySnapshot.docs;\n\n    // Check if the query returned any documents.\n    if (documents.isNotEmpty) {\n      // Get a reference to the document we want to update.\n      final DocumentReference documentRef = documents[0].reference;\n\n      // Get the data (fields and values) of the document.\n      final data = documents[0].data() as Map<String, dynamic>;\n\n      // Check if the specified field ('listFieldName') exists and is a List in the document.\n      if (data[listFieldName] != null && data[listFieldName] is List) {\n        // Create a copy of the current list in the document.\n        List<dynamic> currentList = List.from(data[listFieldName]);\n\n        // Check if the provided index is within the range of the list.\n        if (index < currentList.length) {\n          // Update the value at the specified index in the copied list with the new value ('newValue').\n          currentList[index] = newValue;\n\n          // Update the document with the modified list field using the 'update' method.\n          await documentRef.update({listFieldName: currentList});\n          print(\"List updated successfully\");\n        } else {\n          print(\"Index out of range\");\n        }\n      } else {\n        print(\"The field '$listFieldName' does not exist or is not a list\");\n      }\n    } else {\n      print(\"Document not found\");\n    }\n  } else {\n    print(\"uid not set\");\n  }\n}

Fill free to ask any questions you might have.
Hope it helped 👍

Updating List Field in a Document