Simple Machines announces the release of SMF 1.1.9 and 2.0 RC1-1, addressing a few vulnerabilities found in SMF 1.1.8 and 2.0 RC1. We strongly recommend that all users upgrade as soon as possible. You can see more detail here
I am using SMF 2.0 RC1 and I have to upgrade to 2.0RC1-1 for a security reason. I try to click to upgrade menu in the administrator section but it have failed. Finally, I find out the best way to upgrade:
1. Download the corect upgrade package from: http://custom.simplemachines.org/upgrades/ .In my case, it is SMF 2.0 RC1 to SMF 2.0 RC1-1 — Download
2. Go to the package manager and browse to that package and apply mod.
Now you have upgraded to the newest version.
Have fun !