要编辑AWS SNS主题策略，您可以使用AWS SDK for .NET中的AmazonSimpleNotificationServiceClient类。下面是一个示例代码，演示如何使用C#编辑AWS SNS主题策略：

using System;
using Amazon.SimpleNotificationService;
using Amazon.SimpleNotificationService.Model;

namespace SNSExample
{
    class Program
    {
        static void Main(string[] args)
        {
            string topicArn = "YOUR_TOPIC_ARN";
            string policyJson = @"
            {
                ""Version"": ""2012-10-17"",
                ""Id"": ""MyTopicPolicy"",
                ""Statement"": [
                    {
                        ""Sid"": ""AllowAll"",
                        ""Effect"": ""Allow"",
                        ""Principal"": {
                            ""AWS"": ""*""
                        },
                        ""Action"": ""SNS:Publish"",
                        ""Resource"": """ + topicArn + @"""
                    }
                ]
            }";

            var snsClient = new AmazonSimpleNotificationServiceClient();

            var request = new SetTopicAttributesRequest
            {
                TopicArn = topicArn,
                AttributeName = "Policy",
                AttributeValue = policyJson
            };

            try
            {
                var response = snsClient.SetTopicAttributes(request);
                Console.WriteLine("Topic policy updated successfully!");
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error updating topic policy: " + ex.Message);
            }
        }
    }
}

请确保将YOUR_TOPIC_ARN替换为您要编辑的AWS SNS主题的ARN。然后，您可以使用policyJson变量来设置新的主题策略。在此示例中，我们允许所有人发布到该主题。

以上代码将使用AWS SDK for .NET创建一个AmazonSimpleNotificationServiceClient对象，并使用SetTopicAttributes方法来更新主题的策略。如果更新成功，它将打印出"Topic policy updated successfully!"，否则将打印出错误消息。

请注意，您需要在运行此代码之前安装AWS SDK for .NET并在项目中引用相关的程序集。