After I upload new php, how do I alter its content in dashboard

First of all, someone else built my site with wordpress but I am now responsible for any changes I want made so therefore I need help.

I have uploaded a php file that I copied from one of my other pages. So I know the page code is correct.

-I can see it in the APPEARANCE>EDITOR.
-When I create a new page, I can also select the template from the drop down TEMPLATE menu.
-BUT the editing boxes( i.e. “add media”, content, subcontent boxes, etc,) are not there as they are in the previous copied page.

How can I make the editing boxes show up in this new WordPress page so that i can change the content and therefore make the slightly page different?

Thank you in advance for any help.