Unable to run upgrade script

[Billy_Bob]

upgrading from an older version of MYBB (5 steps) to the current version.  Files are uploaded. I am unable to run the upgrade script. triple checked the path but I get a 404 error- not found.  When I try to open the forum, I get the mybb error of needing to run the install/upgrade process.  I cant even get into the admin.php.  What am I missing?

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I have moved the upgrade file to the system root folder and now i get a 500 error message web site can not handle the request. It appears it can see the file but is unable to go further. ARGGGGG