To transfer a site from a subdomain to the main domain within one hosting account:
clear the contents catalog
main domain and move the entire contents of the subdomain directory to it (or simply rename the subdomain and main domain directories by swapping them).
If the site is bound to a domain address, replace the subdomain address with the address of the main domain and correct the paths in the site's configuration files and / or its database (instructions: WordPress
Since the transfer is performed within the framework of one hosting account, there is no need to transfer the database and change the settings for connecting to it.