I want to migrate my subdomain to replace my main site in the best possible way

I’m fairly new on the wordpress world and I started to test a new e-commerce site on a subdomain. Now I want to swap them or just replace my old site with the new version located in my subdomain (both sites are just a WIP right now). The link posted is my subdomain.

I need some help to understand the best way possible to avoid problems in the migration process with my accounts on each of them (main and subdomain) and just in general to avoid any critical errors. I want my subdomain website to be in the root of my site so people can reach it when they enter the main adresss.

Thanks in advance.

The page I need help with: [log in to see the link]