Click over where it says “New Diagram” on the top left of the Editor window.

 

Once the text field is activated and the blinking cursor is shown, you can type in a new title.

 

Remember to save your diagram to preserve these changes.

More information on naming a sheet or diagram on Editor versions before 6/27/18: https://support.cacoo.com/hc/en-us/articles/207282727-Naming-a-sheet-or-diagram