public function update( $key = null, $value = '' ) {
		if ( $key ) {
			$data         = $this->get();
			$data[ $key ] = $value;

			/**
			 * Filter data before update user data
			 *
			 * @since DozentLMS 1.0.0
			 *
			 * @param  array  $data  User Data in array
			 * @param  int  $user_id  = $user_id
			 */

			$data = apply_filters( 'dozent/user_data/update', array_filter( $data ), $this->user_id );

			update_user_meta( $this->user_id, '_dozent_user_data', $data );
		}
	}