0. optional (i not did it) - in admincp/options.php?do=options&dogroup=onoff enable maintenance mode
1. backup files and mysql
2. in the upload folder (files of a new vbulletin version) rename folders like admincp, modcp to match your custom naming in your current installation (in includes/config.php one can setup custom name for these folders)
3. upload /upload/ folder contents and replace existing files (you have backup)
4. delete includes/xml/product-vbblog.xml file if not using vbulletin suite blogging system, as it prevented upgrade causing following error when i left that file in place:

"Module: vbblog, Step 1 Error.

Mismatch between product version 4.2.0 and vBulletin version 4.2.5 Alpha 2

Were all of the files for this release uploaded?"
5. Go to /install/upgrade.php. It required PHP 5.5+ (vB 4.2.5) and found many mismatch files, i proceed and it completed the upgrade
6. Go to admincp and delete /install directory

and it works