Since you’re using WordPress, I recommend to set up your .com domain as a new domain and then move the domain name to it.
The first step is to add your domain to your account:
The second step is to move your website to your new domain:
Since you’re using WordPress, you’ll also need to update your website URL. WordPress has information on how to do that:
Once the website is fully migrated to your new domain, you can choose to either remove the free subdomain from your account or setup a redirect to send all visitors to your .com domain.