Here is how you can change the page titles that show up in the menu on your Wix Website. I have run into the issue of renaming a page name with a simple Case change not "saving" when I changed a page name.
By this I mean I am renaming the page with the exact same name, but changing to All Caps, Title Case, or Lowercase. Sometimes I found that it doesn't always save my Case-only change, and it would still show the all caps or lowercase version of the page name in the menu, even though I changed it multiple times. So here's a simple solution to get around it – just name the page name with something different, such as add a space, then rename it again with the proper title. This will get it to "stick" with case changes.