Plesk 10.x and 11.x are not available at Home > Updates screen (in the Help & Support group). It is not possible to upgrade to newer version of Plesk.
Update interface in Plesk for Windows 9.5.5 or older does not support new format of Plesk installation repositories. It is not possible to update to Plesk 10.x or 11.x through Updates interface in control panel.
In order to upgrade Plesk 9.x to versions 10.x and 11.x, download latest Autoinstaller from Parallels website and execute it on the server.