Migrating Website from subdomain to main domain

For revamping our website https://www.domain.com, we created a subdomain

exp.domain.com. What steps do I have to follow for the content on the subdomain to replace content on the main domain.

In a failed attempt to migrate content, I moved all the files from exp.domain.com to webroot. However, all my images uploaded in the “Media” section of WP stopped showing up after the migration.

To fix that problem, I installed a plugin called velvet blues in which i redirected the old URL – exp.domain.com (of the development site) with the required url http:www.domain.com. However, the link structure changed completely. instead of https://www.domain.com it changed all the links on the website to http//www.domain.com//xyz (notice that it removed “:”) and added an extra “/”.

I am not sure if this happened after we redirected the URLs using this particular plug in.

It would be great if someone would pinpoint the exact files that need to be changed/preserved for the successful transition of the website.

The original domain is a hard-coded PHP website. Our main aim of creating the subdomain was to have it on WP.