Public Function UpdateMappedPage ( _
    key As SPWebApplication..::..SPCustomPage, _
    newValue As String _
) As Boolean

Dim instance As SPWebApplication
Dim key As SPWebApplication..::..SPCustomPage
Dim newValue As String
Dim returnValue As Boolean

returnValue = instance.UpdateMappedPage(key, _
    newValue)

public bool UpdateMappedPage(
    SPWebApplication..::..SPCustomPage key,
    string newValue
)

using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.SharePoint;
using Microsoft.SharePoint.Administration;

namespace MapCustomAppPage
{
    class Program
    {
        static void Main(string[] args)
        {
            using (SPSite site = new SPSite("http://MyServer"))
            {
                //Get a reference to the web application.
                SPWebApplication webApp = site.WebApplication;

                //Update AccessDenied application page to AxsDnd.aspx.
                webApp.UpdateMappedPage(SPWebApplication.SPCustomPage.AccessDenied, "/_layouts/SubFolder/AxsDnd.aspx");
                webApp.Update();

                //Output the newly assigned application page.
                Console.Out.WriteLine(webApp.GetMappedPage(SPWebApplication.SPCustomPage.AccessDenied));

                //Reset the mapping to the default application page.
                webApp.UpdateMappedPage(SPWebApplication.SPCustomPage.AccessDenied, null);
                webApp.Update();

                Console.Out.WriteLine("Press any key...");
                Console.ReadKey();
            }
        }
    }
}

Imports System
Imports Microsoft.SharePoint
Imports Microsoft.SharePoint.Administration
Module Module1

    Sub Main()

        Using site As New SPSite("http://MyServer")
            'Get a reference to the web application.
            Dim webApp As SPWebApplication = site.WebApplication

            'Update AccessDenied application page to AxsDnd.aspx.
            webApp.UpdateMappedPage(SPWebApplication.SPCustomPage.AccessDenied, "/_layouts/SubFolder/AxsDnd.aspx")
            webApp.Update()

            'Output the newly assigned application page.
            Console.Out.WriteLine(webApp.GetMappedPage(SPWebApplication.SPCustomPage.AccessDenied))

            'Reset the mapping to the default application page.
            webApp.UpdateMappedPage(SPWebApplication.SPCustomPage.AccessDenied, Null)
            webApp.Update()

            Console.Out.WriteLine("Press any key...")
            Console.ReadKey()
        End Using

    End Sub

End Module