Javascript increasing a value and the velocity on increasing it

I need to increase a number and I used setInterval(Function, time) so I put a variable for time: time = 1000 now I need to change it so I put a function that changes it when I click a button:

function changetime() {time = time - 100;}

but it seems that you can't change the time of setInterval while is working... how can I do that? I tried with a setTimeout but the number now changes "jumping". is not regular... I'm not sure but it seems that the "jump" changes when I change the setTimeout time... like if the timeout is now in the setInterval time.

Original code---_>

var time = 1000;
function interval() { setInterval(Function, time);}
function changetime() {setTimeout(interval, 10);tempo = tempo - 200;}