Setting the ROOT PAGE

I changed my main PAGE but, I named the new page I was working on the same as my original main page. Now, I am ready to make the new page the main page for my audience to see but, I have two problems.

1) I do not know how to “retire” the old page and make the new page the one which everyone will see. Where and how do I make this change in WP?

2) My new PAGE was named the same as the old page, WP shows that I have a duplicate and the url looks like this: http://mainurl.com/mainurl It should just be http://mainurl.com

I’ve put the old PAGE in the trash and want to use the new updated page as my main page.