Change page name - new Google Sites
Open the Pages panel on the right
Find the page you want to rename and use the three-dot menu and choose Properties (or double-click on the page name)
Enter the new page name
Optionally customise page path
Use the Done button.
When you create a new page the page name is used as the text in the page header, but this is not connected in any way so this text will not change (just edit the page header separately).
The page name is the main piece of text used in the HTML <title></title> tag: if you have a site name then Google Sites makes the HTML <title></title> tag in the following format:
<title>Site name - Page name</title>