Enable Media Replace – ERROR Message

Trying to install this plugin so I can update imagery on my WP 3.5.1 site and I get this sequential message:

Downloading install package from http://downloads.wordpress.org/plugin/enable-media-replace.zip…

Unpacking the package…

Could not create directory. /var/www/wp-content/upgrade

Any ideas as to why this is coming up? Thanks for any suggestions.

http://wordpress.org/extend/plugins/enable-media-replace/