Tara on "Error 500 when updating to 4.2.1"

this is an error I'm receiving only when I try to update to WordPress 4.2.1 or update plug-ins.

WordPress update:
try MANUAL updating. Download a fresh copy of the WordPress .zip file to your computer, unzip it, and use that to copy up all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory. You may need to delete the old copies of files & folder on your server before uploading the new ones. Read the Manual Update directions first.

Plugin update:
manual installing of plugin: https://codex.wordpress.org/Managing_Plugins#Manual_Plugin_Installation