First, grab the update http://download.windowsupdate.com/ms...e119de2ccd.cab. It's a cab archive, so don't extract it, just leave it as it is.
Then download this tool and install the x32 or x64 version according to your configuration.
Now download this one and simply extract it in a folder. Connect your Lumia to Zune and wait for the sync process to end. Close Zune. Put the .cab file in the same folder than the tool called WP7 cab sender, at the root of the file "WP7 Update Cab Sender.bat", it's important.
Launch the batch file "WP7 Update Cab Sender.bat", an msDos window should open, asking you to choose a letter for an option: choose the option of your choice but don't forget to make a backup before installing the new update.
Then you only have to wait for the backup / update to end. Don't close the MS-DOS window before the end of the process. It's done, your phone is now running the new update.