Backing up a Simple Machine Website requires two simple steps. You need to backup the files and then to backup the database.

Backing Up the Files

You can download all the files usingyour favourite FTP software. This can be done using Filezilla or WS-FTP-Pro from Ipswich. There are lots of FTP software around and they will all be suitable for the file backkup. A second approach to backing up files will be to use the cPanel tool.

Backing Up Files in cPanel

You can simply select all files and click the "compress" button in cPanel. Choose a name for your file and select the zipped file option. Now proceed to compress the file. This will create a file backup in your public_html folder or in any destination of your choice in your website control panel.

Source: https://www.todhost.com/knowledgebase/755/How-to-backup-a-Simple-Machine-Forum-website.html
Backup the Database

There is a database backup tool in cPanel which makes this process easy. That is the phpmyadmin tool. phpMyAdmin can be found under the database section in your cPanel environment. Click on the phpMyAdmin tool and select your database - the database you want to backup and click the export button to download your prefered database. You can use the phpMyAdmin tool to restore your downloaded database.

Backing Up With SMF Tool

You can backup your website database from within the SMF website dashboard. This wil be after you have successfully logged in with the required information for authentication. This can be done with the “Backup Database” option in the Administration Center of your SMF forum Admin > Forum Maintenance tab.

Make sure you save both structure and data of the database as well, either in two different steps or in one unique save.

It is always recommended to have backups created on weekly or at least monthly bases. Backups age good so that if your website have issues, it can be restored back to the point when it last functioned effectively without error.