Under tutorials section there is something but essentially you can use system 'save' and 'load' which is just like a save slot, and for example the user's highest ever score would be stored in some global variables.
Whenever a player finishes a game, with events you could compare the end score variable that you have now with their high score variable and if the new one is faster then update the high score variables, and then 'save' to a save slot. Whenever the user launches the game again you 'load' from the save slot.
Or you can save the score to local storage, this is like saving and loading single data rather than the full game. The method depends on what is more useful for your type of game.