In order to create a backup of your store's database you need to log in your OpenCart admin area.
Go to System » Backup/Restore
In the "Backup" box you can see all the tables of your store's database. Click "Select All" to add them to the backup.
Then click the "Backup" button on the upper right corner of the page.
This will create a backup of the database and let you download it to your computer for safe keeping.
Now you have a backup of your database's current state which you can restore later.
In order to restore a backup you made previously from the OpenCart admin, click on the browse button to select the backup.sql file from your computer.
Now click the "Restore" button on the upper right corner of the page.
This will overwrite your store's current data with the backup copy you selected