At first to backup your sites' files and databases from terminal use the commands:
Database backup:
mysqldump -u database_username -p database_name > site_database.sql
Fill in the "database_username" with the site's database username, "database_name" with the site's database name and the "site_database" with the name of the exported database.
Compress files:
tar -zcvf site_files.tar.gz /path/to/files/folder/
Fill in the "/path/to/files/folder/" the path of the site's folder e.g. /var/www/domain.com/html/
When you're ready, download databases and files to your PC.
Then, when your hosting company reinstall CentOS 7 with Direct Admin, you can go through this guide and complete the necessary steps.
Finally, to restore your sites, simply upload the .tar.gz compressed files to the site's public folder and uncompress. To restore database you can go through phpMyAdmin or via terminal with the bellow command in the folder with the uploaded databases:
mysql -u database_username -p database_name < site_database.sql