Sorry that's a little different. The easiest thing to do is place a file called app_offline.htm in the root of your website. You can have any content in this file and it will be picked up by IIS automatically to reroute any requests from that site. When you are done, simply remove or rename the file.

Another consideration is to copy your site to a new folder and work on the upgrade there. When you are ready to launch you can use IIS to point to this new folder. If anything goes wrong you can point IIS back to old site and keep the old site running while you continue working on the new one.

The most important issue is to backup your site and backup your data before making any changes.

He who laughs last made a backup.

I hope this is of some help to you.