system -> compare variable -> score < highscore -> lastscore = current score.
This will work, first you check if your current score is lower then the high score if its true, then you set the current score to the last score and you display that. The event should go in the end of each level/attempt best to be called in a function.
And well of course then the current score is higher then the high score then you change both, this can be done using another event or an else statement in the same one.