Log into your website's cPanel and then click on the icon that says "backups". Then you will see a square button that says "Download or Generate a Full Website Backup". Click on that and then a backup page will appear.
Make sure your backup destination is "Home Directory" and click Generate Backup. This may take a few minutes. When it's done, your backup file will appear under "Backups available for download" on the same page. You will have to either refresh or go back into this page for it to come up. Then download the file to your Dropbox or computer!
I would make a practice of doing this after every time you make any website changes. This can also be automated and scheduled by using a plugin call backup buddy: