creating under a subdomain and then moving to main domain

We created new site on wordpress while leaving the old one up , so I created a subdomain (blog.mywebsite.com) and set up the a-record for the new folder and subdomain and successfully built the site under the subdomain while leaving the other site intact. When I was ready to take down the old one, I changed the dns settings of main domain to the new wordpress site. When you go to the main domain everything is fine, you see the new site but when going to any other pages, you still see the blog.mywebsite.com. We are trying not to lose the ranking from the original site and also have broken links (I just kept the page names the same as the old site to keep it simple and did a custom permanlink so the urls would match to the old pathways)

Someone told me I could go into the general tab and just change one of the lines to the main domain and it will fix this issue. First I am not sure which line (blog address or url address) and I vaguely remember trying this once and the site wouldn’t work anymore. I know this is prob. something really simple, I just need to know how once you are done creating your site on the subdomain , how you get it to go to the main domain without having to go in and manually make changes in the tables.

thanks!