Split Data from Wordpress Database
That's really two operations: splitting the combined db to separate WP and Civi ones, and migrating.  For the migration part see the manual.

To split the db, you could make a duplicate of the original as the Civi db (dump and load), then in the Civi one delete all the non-civi tables and in the original/WP one, delete all the civi tables.  In your civicrm.settings.php file find the setting for CIVICRM_DSN and change it to use the new database name.

Make sure you keep a backup of your original combined db just in case!