You can export your joomla.com website using the Export Site link, located in the options menu (gear icon) for the specific site.
You will get a confirmation message that includes the e-mail address where the download link will be sent.
You will receive an e-mail with the download link to your site data, a username and password.
After visiting the provided link, you will be prompted to log-in. Use the credentials inside the e-mail you received. After authentication, the download of your site data will begin. The archive includes your Joomla! database and files.
NOTE: If you do not receive your export e-mail within an hour, please check your spam folder, or your Promotions tab if you are using Gmail.