How to update Azure App Configuration Settings using Code (.Net Core)

I have an Azure app registered on Azure portal which is created in .NET Core 2.0 This app reads some config value from the Application settings section from the portal as shown in below image.

Now at some stage I want to update those config value from code. I have searched for many article but not found how to update Azure Application settings from code. Can any one have an idea or suggestion that How can I update Azure Application settings using .NET Core 2.0 C#?

This can be accomplished using Azure.ApplicationModel.Configuration nuget package.
In particular, the SetConfigurationSetting method seems to do what you are after.

string connectionString = "<connection_string>";
var client = new ConfigurationClient(connectionString);
ConfigurationSetting setting = client.SetConfigurationSetting("some_key","new_value");

Note: This package is in preview

You could use c# to call the REST API Web Apps - Update Application Settings manually.

PUT https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Web/sites/{name}/config/appsettings?api-version=2019-08-01

For more details about how to call Azure REST API in c#, you could refer to this link.

If you want to use c# to do it, you could try with Microsoft.Azure.Management.Fluent package, the below is the sample code, you could have a try.

            string tenantId = "*******";
            string clientSecret = "********";
            string clientId = "********";
            string subscriptionId = "*******";
            
            var azureCredentials = new AzureCredentials(new
              ServicePrincipalLoginInformation
            {
                       ClientId = clientId,
                       ClientSecret=clientSecret
            }, tenantId, AzureEnvironment.AzureGlobalCloud) ;
            var _azure = Azure
           .Configure()
           .WithLogLevel(HttpLoggingDelegatingHandler.Level.Basic)
           .Authenticate(azureCredentials)
           .WithSubscription(subscriptionId);

            var appResourceId = "/subscriptions/**********/resourcegroups/*******/providers/Microsoft.Web/sites/***"; //Get From WebApp -> Properties -> Resource ID

            var webapp = _azure.WebApps.GetById(appResourceId);

            webapp.Update()
                .WithAppSetting("test", "testvalue")
                .Apply();

The library has been changed to Azure.Data.AppConfiguration.
Azure.ApplicationModel.Configuration is depracated

This is an addendum to George Chen's answer.

To avoid the "Operation returned an invalid status code 'Forbidden' exception after calling _azure.WebApps.GetById(appResourceId), you need to ensure the service principal associated with the Azure Credential has contributor access to the subscription the web app is in. For more details refer to https://learn.microsoft.com/en-us/azure/active-directory/develop/howto-create-service-principal-portal.