A question before I install

If the old domain is still there, you can avoid using any plugins and just add this directly to a .htaccess file in the root folder under the old domain:

Redirect 301 / https://mynewsite.com/

This will redirect the entire site.

Thanks for this @manndtp.

How will it work if I want to delete the existing site?

I am moving my blog from https://blog.capeholidays.info to just https://capeholidays.info

Thanks.

To redirect requests for sub.yourdomain.com/uri to yourdomain.com/uri:

RewriteEngine on
RewriteCond %{HTTP_HOST} ^sub.yourdomain.com$
RewriteRule ^(.*)$ https://yourdomain.com/$1 [R=301,L]

@manndtp kind of you. Thanks.