Backing up the database of your forum is easier than ever with phpBB3. All you have to do is select the Maintenance toolbar from the top menu in your Administration Control Panel and then click on the Backup link in the left menu. Next, click on the "Select all" link in order to select all tables for the backup and then click on the submit button. You can leave the default settings as they are. In a few moments a download pop-up for your database will appear.
If you have a big forum with a lot of users and posts the backup might take some time to be created.