There is probably something wrong with the file permissions. Make sure that files and directories are owned by the user www-data or apache (or whatever your hosting provider tells you it should be) and check that all the files are uploaded and has the correct permissions.
Sometimes, especially if you use Windows, file permissions can be changed because the file system used by Windows doesn't understand them. That's why it's always easier to use a Linux or Mac or BSD or Solaris computer when uploading to a server running any of those operating systems.
Also, the file name is upgrade.php, not update.php.