Can't upgrade to vB 5.0 from "vBulletin® Version 3.7.3"
 

Upgrade says file is missing " /core/install/includes/class_upgrade_373.php "
Ver. 373 is nowhere to be found from any version..
If I try to upgrade to ver. 3.8.7 Patch Level 3 to then upgrade to ver 5. same file is missing.
Same with ver 4

Where can I get this " class_upgrade_373.php "
or how do I upgrade vBulletin® Version 3.7.3 to Ver. vb5_connect_5-0-0_Beta_17

Please help!