Open your SSH/Telnet client and log into your website. The command line prompt you will see will vary by OS. For most hosting companies, this will bring you into the FTP root folder.
You can either change directoties to wherever the backup is located and type in the following:
mysql -u dbusername -p databasename < backupname.sql
Or if you do not want to change directories and you know the path to where the backup is located, type in the following:
mysql -u dbusername -p databasename < /path/to/backupname.sql
You will be prompted for the database password. Enter it and the database will backup.
If your hosting company has you on a remote MySQL server, such as mysql.yourhost.com, you will need to add the servername to the command line. The servername will be the same as in your config.php. The command line will be:
mysql -h servername -u dbusername -p databasename < backupname.sql
mysql -h servername -u dbusername -p databasename < /path/to/backupname.sql