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

 

https://cacoo.com/diagrams/FUrIR6presRse4nL-AF50C.png 

 

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