Hi,
what you need is a way to save data on a per scene base, and for this, you can use PlayerPrefs.
each scene would have specific PlayerPref Key, like "Level X" x would be 1 2 3 depending on the scene.
then you end up with playerprefs like so:
"Level 1" -> 4
"Level 2" -> 9
"level 3" -> 2
4, 9 2 being made up values for each of your levels score.
then, when you want the total, either you compute it on the fly, or you go through all playerprefs and add them in turn.
Bye,
Jean