Wrong page names displaying in address bar

I changed the names of some of my pages before the site was done, and now that it’s done, I just went to change the permalink structure to display the page name instead of the question mark/id number.

The problem is that now, once I click on a page, the old page name displays in the address bar.

Example:

I click “classes” so the address bar should read: https://www.example.com/classes

Instead it reads it’s old title: https://www.example.com/academics

I tried changing the individual permalink on each page, but then all of the pages come up with a 404 not found.

Any ideas on this?

Thanks for reading.