First step is to take backup for both the folders (i.e main public html as well as sub domain folder).
Then, move the files inside the sub domain into the main public_html folder (i.e Document Root). This will load the WordPress site from the main domain public_html folder when you access the main domain URL
PS: Please note sometimes it would override files in the main public_html folder. So its good you talk to a sysadmin and have him get it done. Its fairly an easy task and wouldn’t take much time, but done by an expert is the best way forward.