Open the console with F12 then type SugarCube.State.variables.variablename=bla
You'll get predictions and can tab complete, strings need to be quoted, array elements (not sure there are any in this) can be accessed with .variablename[number].anothervariablename=bla
Variables aren't updated by SugarCube until you click a link to load another page, any page at all (eg. opening and closing the phone, with its exit button, would work), pressing the back button to the page you made the change on would undo the change as well.