If your website has UpdraftPlus installed, once you are logged into your website and are on the dashboard, click on UpdraftPlus in the top navigation menu. From there, click on backup now and follow the prompts. 


If your website is hosted on Pantheon, then your website is automatically backed up each day. If you need to do an additional back up, this can be done via your Pantheon Dashboard: https://pantheon.io/docs/backups/