If I got you right, then define the health
var health:Number = 100;
You wanted the health to randomly decrease or increase, so we'll have to use the random() method. So when the button is pressed:
health += Math.floor(Math.random()*20-10);
This will add the health number -10 to 10 randomly. Math.floor should fix 'ugly' numbers.
if you don't want the health to get under 0 or above 100, use conditional stuff after changing the health var,
if(health>100){
health = 100;
}else if (health<0){
health= 0;
}
Hope this helps.