Cannot Upgrade to 5.5.2 from 5.5.1
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.
(your site root)/updates/concrete5.5.2
Then go to Dashboard > System & Settings > Backup & Restore.
The URL method is really only necessary when your current core is somehow broken / not working and you can't get to the dashboard.
Appreciate the assist! Perhaps it's time to update the INSTALL file in the archive to reflect that procedure.
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.
Clean / Disable Cache
Backup files & database
rm concrete ; ln -s ../c5XX concrete
I upgraded one today from 5.5.1 to 5.5.2, and it said already upgraded to 5.5.1 - several other people on IRC mentioned getting the same message.
After hitting retry *three* times it finally said upgraded to 5.5.2 - the site is running great.
Methinks something else might be going on...