Here's a solution to your question:
- Download all files to your computer (via ftp)
- Export the mysql database to you computer via phpmyadmin (inside cpanel)
- Upload all files to httpdocs dir (on new server with plesk)
- Create new database in plesk
- Import the database file via phpmyadmin.
Do not forget to create same database user (with the same password) as it was in cpanel or create different user with different password but then change the credentials in wp-config.php file after you uploaded everything to httpdocs dir.