I would like to do the same thing - upgrade my current one-click installation of mediawiki 1.3.x to the latest 1.5.2. Did you ever figure out how to do this?
As far as I can tell, it goes something like this:
1. download the archive of the files for the new version
2. expand the new files either into your existing installation, or into a clean directory (where you would need to copy your existing configuration into)
3. perform any database and config file upgrades
I got as far as step 3, trying to run 'php update.php' to upgrade my database schema, but it looks like it's trying to log in as my shell username, not the database username. I couldn't figure out how to change that as it's not explained in the README or UPGRADE documentation and I don't have time to sort through all that php code.