Hi
First of all you need to be sure that you have created the environment for WordPress to run.
1. Download and install WAMP Server / XAMPP. Installing this will get Apache, MySQL and PHP installed on your local machine.
2. Download Worpress folder by Login to your sites FTP.
3. Copy this folder to your www folder in WAMP / XAMPP. Rename it with “yourblog”
4. Add DB details of online site to wp-config.php in local installation. Make sure you backup this file before editing.
5. Launch your browser and type localhost/yourblog
Your local installation is up and running.
Once you have done the desired changes to your website upload the contents to the server via FTP. Make sure you restore the wp-config.php file to the original values.
Hope this helps.
Cheers