https://www.akeebabackup.com/products/admin-tools.html
or
Or you can place the following code in the .htaccess file located in your Joomla's /administrator folder:
Order Deny,Allow
Deny from all
Allow from x.x.x.x
Be sure to replace x.x.x.x with your IP address. If the file .htaccess does not exist, you can simple create one using a text file editor.