John Godley

Enhances the WordPress media library by allowing you to rename the underlying media files. Any change to a media item’s title will result in the media filename also being changed.
See the screenshot for more details of where to rename media files.
Note that all thumbnails will also be renamed. If you are using a caching plugin, such as WP Super Cache, then you will need to clear your cache so that any references are updated. This plugin does not update existing direct links to files (links inside a WordPress gallery will change).

Rename Media

Rename underlying media files from the WordPress media management interface

AboZain Albanna

Rename Default Post is a simple plugin gives you the ability to Rename your Default Post label to custom name in admin menu. 
Why would I leave my client a menu item that says ?Posts? when they are adding news updates, or portfolio items, 
 So you can simply use this plugin.
<h3>For Example Change Posts to News</h3>
How the plugin works: = 
Go to Settings -> Rename default post.

Rename default post Labels

Awesome

A simple, lightweight WordPress plugin Rename your Default Post labels to custom name in admin menu

Rename Media vs Rename default post Labels