Upgrade in Dashboard tells me I'm already at the latest version (5.5.1) and doesn't do anything.
I also manually downloaded the 5.5.2 file package and copied it over the top of my existing installation. I used the /tools/required/upgrade.php tool per the upgrade instructions in the downloaded ZIP file. It too tells me that I'm already at the latest version and asks if I want to run the script again anyway. After running it says I'm upgrade to 5.5.1.
I took a peek at the upgrade instructions in the INSTALL textfile and while they *should* technically still work, you're right that the last time they got much attention was likely before the whole dashboard upgrade feature was introduced. We'll add revising those to our to-do list for next time.