Upgrading Wordpress on IIS 5 is very easy: all the user has to do, is to unpack the contents of the latest official Wordpress distribution file to the folder/directory where Wordpress is already installed, choosing to OVERWRITE all files in the way.
Despite the fear that a total overwrite might cause, it is a safe operation, because there is only one critical file (wp-config.php), that could cause problems, if lost, but is distributed with a different name (wp-config-sample.php), to be renamed only on first time installs.

With IIS 6, things aren’t so easy.

The system running IIS6 should have “simple file sharing” disabled. Such option allows the “security” and the “web sharing” tabs to appear, on the dialog box that shows, when selecting “folder properties”.

Knowing this, after OVERWRITING all Wordpress files, as if upgrading Wordpress on IIS 5, to upgrade Wordpress on IIS 6, just follow the next six steps.

Step #1 – Locate the Wordpress folder on the local file system (for example, c:\webs\site\wordpress);
Step #2 – Choose properties (ALT+ENTER) for that folder.
Step #3 – On the security tab, select the “Internet Guest Account”, then click the button “advanced”.

Wordpress upgrade on IIS 6 – Picture 1 of 5

Wordpress upgrade on IIS 6 – Picture 1 of 5 – select the “Internet Guest Account”, then click the “Advanced” button.

Wordpress upgrade on IIS 6 – Picture 2 of 5

Wordpress upgrade on IIS 6 – Picture 2 of 5 – Check both the checkboxes, then press ok.

Step #4 – As shown on the picture above, select both check boxes, to (1) allow inheritable permissions from the parent folder, and (2) to replace the previous permissions, everywhere down the tree. Then, press OK.

Wordpress upgrade on IIS 6 – Picture 3 of 5

Wordpress upgrade on IIS 6 – Picture 3 of 5

Step #5 – Point the Internet browser to the site’s Wordpress URL (for example, http://arturmarques.com/wordpress) and follow the instructions on screen.
As the image above shows, you’ll first have to choose to upgrade the database.

Wordpress upgrade on IIS 6 – Picture 4 of 5

Wordpress upgrade on IIS 6 – Picture 4 of 5 – Upgrade goes on

After upgrading the database, some files will also be updated.

Wordpress upgrade on IIS 6 – Picture 5 of 5

Wordpress upgrade on IIS 6 – Picture 5 of 5 – The conclusion

Step #6 (final) – After a few seconds, the upgrade is done. Just visit your Wordpress site to confirm.

I hope this guidance can help someone who gets stuck on something so simple, yet with the potential to cause a major headache.

UPDATE: some Wordpress upgrades change the database structure. If so, it will be needed to upgrade the database, by visiting http://server/path/wordpress/wp-admin/upgrade.php
where server and path should be replaced by proper values, specific to each installation.

For example, here at home, the upgrade can be invoked the following way:

http://localhost/cgi-bin/wordpress/wp-admin/upgrade.php