WordPress migration

Hi there, I need some WordPress help!

I have designed a new wordpress site that needs to be migrated to an existing (old) wordpress site. I designed it under a different domain name. It was all finished so I was ready to migrate. I used the plugin All-in-one migration and exported the new design file. And a copy of the old site backup also.

I tried on the new site to add the old domain name in settings… which completely cleared my site and I can no longer login! But… I have a backup file!

Then on the old WordPress site, I tried to reset the database before I import the new file to the site. And I am locked out of the site and cannot login!

Stressful! Clearly I don’t know what I am doing! Can someone please help or advise me what to do next?

Many thanks
N

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