String updateObjStr = [{"fields":{"Upside_Amount__c":"1000","Id":"a0N21000002l2e7EAA"}},
{"fields":{"Upside_Amount__c":"2000","Id":"a0N21000002raT7EAI"}},
{"fields":{"Upside_Amount__c":"300","Id":"a0N21000002raUFEAY"}}];
List<Map<String, Object>> updateString = new List<Map<String, Object>>();
Map<String, Object> deserialized = (Map<String, Object>)JSON.deserializeUntyped(updateObjStr);
for (Object instance : (List<Object>)deserialized.get('fields')){
updateString.add((Map<String, Object>)instance);
}
throwing error Invalid conversion from runtime type List to Map