Rewrite and replace url wp-admin/edit.php and wp-admin/post-new.php

The technical answer to this is to put an .htaccess file inside your wp-admin folder with this code:

# Remove .php from pages so you can use /wp-admin/edit
RewriteEngine on  
RewriteCond %{REQUEST_FILENAME} !-d  
RewriteCond %{REQUEST_FILENAME}\.php -f  
RewriteRule ^(.*)$ $1.php

# Use /wp-admin/addnewpost
RewriteRule addnewpost post-new.php  

The practical answer is that this is completely horrible, involves making sure your Apache server parses .htaccess below root, and won’t remove the extension from the links in admin so you’ll still see /wp-admin/post-new.php when you click a link to create a new post.

Above all else, it breaks the way WordPress Admin is intended to be used, and is a BAD IDEA. Please don’t do it.