move the subdomain folder to httdoc,
you adjust in the database the url under ps_configuation, in htccess and wherever some moduls that have the old path,
You could put the subfolders content into the htdoc folder but I prefer a html5 set up in that and use a shop in a folder. That way you can easily change the face of the site whitout having to change the entire shop.
I suggest you read up in the presta documentation before you start and have all data backed up.