Recently I started having some problem on a couple of websites I run that use WordPress as CMS, since WP implemented an automatic upgrade system life became easier for us, no more messing around with ftp clients, overwriting files, deactivating plugins and what not. Was a matter of clicks and in a minute or two our WordPress installation was up to date.
But as I said at the beginning, for some reason this auto upgrade started failing, no php errors or warning, a simple Installation Failed message was all I got.
Goggle results gave me the idea that I wasn’t the only one with this problem, however no solution was at the horizon, sadly a manual upgrade was the only way, now, if you only have one blog that’s just fine but when you manage many it can be quite cumbersome to upgrade each one of them.
Just upload class-wp-upgrader.php to the directory wp-admin/includes overwriting the existing file.
Try again the automatic upgrade, everything should work as supposed. At least for me it did solve my problem. Good luck and don’t forget to backup your files :-)