Interval parameters wont update

1

Hi, I have an interval that executes a function every certain amount of time. This time is supposed to be controled by a variable called “frecuencia”. The thing is, when I change the variable the interval will still use the same original value. How can I make the interval UPDATE after I change the variable?. Here is the code:

 
var Items = new Array(bombon, regalo, corazon);
pos = new Object();
pos._x=random(550);
i=0
itemspeed=5
frecuencia=1000
goal=10
function lvlup(){
 frecuencia-=200
 goal+=20
 level++;
}
function randomize (){
 targeta=Items[random(3)]
 pos._x=random(550);
 targeta.duplicateMovieClip("item"+i,i,pos);
 i++;
}
var Int = setInterval(randomize, frecuencia);

I am calling the function lvl up in a separate part.