First, you will want to add `public float highscore;` with your other variables at the top. Then, you will want to add this code into your start function. This is a very basic save system, and can easily be modified to fit the needs of the game. The comments in the code explain the script in further detail.
// Saves the score and highscore into the save file
score = PlayerPrefs.GetFloat("score");
highscore = PlayerPrefs.GetFloat("highscore");
// Changes highscore to current score if current score is higher
if (score > highscore)
{
highscore = score;
}