jiggs
All you need to do is unpack the plugin, make the change to the file, run php lint test on the modified file (to make sure you don't have a typo, etc.) /path/to/php -l /path/to/file.php, login to the database, go to the _plugin table, find the oauth2 plugin record, update the install_path (to remove the .phar), and set isphar to 0. Then of course the plugin needs to be enabled.
Cheers.