One way to make a maintenance page is to use the app_offline.htm feature of IIS. Using this feature you will be able to show the same html page to all your users notifying them about the maintenance.
There is a nice post here in StackOverflow about it. ASP.NET 2.0 - How to use app_offline.htm.
Another thing you could do is to notify your users that there is a scheduled maintenance so that they also be aware and stop using the application.
That all depends on the time you need to upgrade your application. If the upgrade is to upload the new files and take not more that a minute or two, its most likely that your users wont even see it.