Change the path where wordpress plugins are uploaded

You can change the Plugins directory using constants defined in wp-config.php:

Set WP_CONTENT_DIR to the full local path of this directory (no
trailing slash), e.g.

define( 'WP_CONTENT_DIR', $_SERVER['DOCUMENT_ROOT'] . '/blog/wp-content' );

Set WP_CONTENT_URL to the full URI of this directory (no trailing
slash), e.g.

define( 'WP_CONTENT_URL', 'http://example/blog/wp-content');

Set WP_PLUGIN_DIR to the full local path of this directory (no
trailing slash), e.g.

define( 'WP_PLUGIN_DIR', $_SERVER['DOCUMENT_ROOT'] . '/blog/wp-content/plugins' );

Set WP_PLUGIN_URL to the full URI of this directory (no trailing
slash), e.g.

define( 'WP_PLUGIN_URL', 'http://example/blog/wp-content/plugins');

If you have compability issues with plugins Set PLUGINDIR to the full
local path of this directory (no trailing slash), e.g.

define( 'PLUGINDIR', $_SERVER['DOCUMENT_ROOT'] . '/blog/wp-content/plugins' );

But wait!

If the need to define these paths originates from moving your installation from a subdirectory to the document root, your problem is very likely that you didn’t make that move properly. Please reference this Codex entry about giving WordPress its own directory.