Bad Links After Migration

I recently changed the domain name on a newly built site. I was assured that the temporary site links would change to the new site, but this did not happen. The links still point to the old temporary site which no longer exists.

I was considering doing a find and replace but the plugin I have did not do the job. I am new at this, so I am leery of accessing the database, but short of trying to fix links in over 300 posts, I don’t see another way of doing this. Does anyone have any suggestions?

Thanks for your help

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