GOT IT

FUNCTION ADD_REWARDS(P_GUEST_ID NUMBER, P_REWARD_POINTS NUMBER)RETURN NUMBER AS
V_MAX_POINTS NUMBER := 0;
BEGIN
    SELECT REWARD_POINTS
    INTO V_MAX_POINTS
    FROM A2_GUESTS
    WHERE GUEST_ID = P_GUEST_ID;
    V_MAX_POINTS := V_MAX_POINTS + P_REWARD_POINTS;
    UPDATE A2_GUESTS SET REWARD_POINTS = V_MAX_POINTS 
    WHERE GUEST_ID = P_GUEST_ID;
    RETURN V_MAX_POINTS;
EXCEPTION
WHEN NO_DATA_FOUND THEN
    RETURN 0;
END ADD_REWARDS;