Wordpress migration from sub domain to root - same host

We have a new website currently sitting on a subdomain, we would like this migrated to the root domain, replacing the old website currently there. We do not need the old website backed up, but we do want to be sure not to risk losing the new site, or other sites also on subdomains so it may be necessary to back these up.

The current OLD site https://glasshue.com

The NEW site which needs to be moved to the root https://development.glasshue.com